In addition to what Nobody has already covered:
Sources of advantage:
The Best Solution for Advantage if your DM allows it: Per the optional Xanathar's rules for using Tools and Skills Together:
If the use of a tool and the use of a skill both apply to a check, and a character is proficient with both the tool and the skill, consider allowing the character to make the check with advantage. This simple benefit can go a long way toward encouraging players to pick up tool proficiencies.
Per the PHB (under Tools in Equipment section), musical instruments are a type of tool, and unlike artisan's tools (to which Xanathar's adds special benefits for proficiency), musical instruments get nothing new aside from the baseline improvements for tools, so I'd hope DMs would be more inclined to give you consistent advantage.
So just sink a tool proficiency (acquired from your background, or downtime training) into drums (per your prior post, the item is a drum). You're done. You have permanent advantage on all uses of the drum that overlap with the Performance skill (unless hampered by a source of disadvantage, and there's nothing to be done about that), which should be all of them. Skip the rest of this section and go on to guaranteed bonuses.
If you don't need your concentration for the spell you're casting, or a party member can cast it for you, Enhance Ability (Eagle's Splendor) will give you up to an hour of Advantage on all Charisma checks.
Out-of-combat, Help is a cheap source of advantage. Have a buddy with rhythm beat box a metronome beat to keep your rhythm steady or something, now you've got advantage.
Now that you've got advantage basically all the time, we'll move on to:
Guaranteed boosts:
Expertise (most important!): If you haven't done so already, make one of your Bardic Expertise picks (two each at 3rd and 10th level) Performance. That's a fixed +2 above mere proficiency at level 3 (total with Cha 20 of +9), rising to +6 as you level (total with Cha 20 of +17). Expertise breaks bounded accuracy more easily and more completely than anything else in the game, and helps the most here.
Ioun Stone of Mastery - This straight up boosts your proficiency bonus by 1 for all purposes. Combined with Expertise in Performance, that's a +2. It is a Legendary Item, and every skill-oriented character in the party will want it, so this might be out of reach.
Tome of Leadership and Influence - Very rare (so again, possibly out of reach until later tiers), but since you say in your other post that you've already maxed Charisma to 20, this boosts your score and maximum, so each one of these you use gives a permanent +2 to Cha, and an associated +1 to all Cha-based ability checks. Drawing the Star card from a Deck of Many Things has a similar benefit (though it's capped at 24, unlike the tome), but it's Legendary and random draw, so it's not at all reliable.
There's a couple notable multiclassing cases to mention, even though you mentioned your class is set:
Peace Domain Cleric's Emboldening Bond. You don't even have to be the one to take it; any member of your party (who meets the multiclassing stat requirements) can dip a single level to pick this up, and it's bonkers good both in combat and out. For 10 minutes, up to proficiency bonus allies (the cleric can be one of them) can be joined in a bond that, while they're within 30' of the cleric, grants as +1d4 attack rolls, ability checks, and saving throws (limit once per turn). It's a 10 minute long combo of Bless and infinite Guidance that stacks with both. The bond can be established proficiency bonus times per day, so you'll have access to this benefit 20-60 minutes per day. Out of combat, said cleric can be adding Guidance as well, for +2d4 to your checks.
(Costly on levels, and requires high Wisdom to be useful, but worth mentioning for how ridiculous it can be) Dip three levels of Ranger and take the Fey Wanderer subclass. If you want to be the Face to end all Faces, adding this to Expertise in a Cha-based skill will get you there. The subclass's basic feature allows you to add your Wisdom modifier to all Charisma checks. With an eventual 20 Wis, 20 Cha, and Expertise, you'd eventually reach a +22 to your Performance check (and the maximum possible DC is 28) before any other modifiers.
You'll be doing pretty well with just Cha 20 + Performance Expertise + a consistent source of advantage for awhile, so your out of combat spellcasting is likely to be pretty reliable. Remember, you need a high enough level spell slot to even try to cast spells from this item. If you have Cha 20, and stick with Bard the whole way, the table for minimum rolls, DCs, and risk of failure will look like this:
| Level |
Min Roll |
Max DC |
Max failure % (normal) |
Max failure % (w/adv) |
Guaranteed success spell level |
| 1-2 |
10 |
12 |
10% |
1% |
Cantrip |
| 3-4 |
10 |
14 |
20% |
4% |
Cantrip |
| 5-6 |
12 |
16 |
20% |
4% |
1st |
| 7-8 |
12 |
18 |
30% |
9% |
1st |
| 9-10 |
14 |
20 |
30% |
9% |
2nd |
| 11-12 |
14 |
22 |
40% |
16% |
2nd |
| 13-14 |
16 |
24 |
40% |
16% |
3rd |
| 15-16 |
16 |
26 |
50% |
25% |
3rd |
| 17+ |
18 |
28 |
50% |
25% |
4th |
Your chance of failure with advantage alone on the hardest spells available remains below 10% through level 10, and you'll have full access to all Bard cantrips from the get-go. Remember, you probably won't need this that often in combat. If it's a spell you want to use a lot/reliably-in-combat, you'll just pick it as a known spell (the item can't cast non-Bard spells anyway), so this is likely to be used primarily for rarely needed out of combat utility spells, where, even if you don't have permanent advantage from drum proficiency, you can rely on party members to Help with your performance (and provide Guidance if any party member has it). Between Guidance and advantage alone, you'll do well for quite awhile.
Once that's not enough, the standard character level limits for magic item rarity relax just as the DCs start to get away from you. You can get the Luckstone anytime (Uncommon items are considered appropriate even at level 1), then at level 11 (the first level with >10% chance of failure with advantage), Very Rare items (e.g. the Tome of Leadership for an extra +1 to all Cha rolls of any kind) can help you keep pace with the increasing DCs (and the Tome(s) doesn't even eat an attunement slot!), and at level 17 (with the worst failure chance) Legendary items like the Ioun Stone of Mastery (for a +1 to all proficient skills, and +2 to skills with Expertise) become possibilities.
Even without them, at level 17+ having just one of Guidance or Emboldening Bond in effect, on top of advantage and expertise, will reduce your risk of failure on 9th level spells to 14.38%; if you have both, the risk of failure drops to 6.88%. Add on the effects of any of the three fixed bonuses available from magic items (+1 per Tome, +1 from Luckstone, or +2 from Ioun stone) and you'll get very close to guaranteed success.
But if almost guaranteed success isn't enough, there's one last approach you haven't considered:
Setting a floor on your rolls:
Since your goal is hitting a fixed DC, not "the highest possible roll", there's a third option to consider here, namely the rare spells and game features that either set a lower limit on your roll, or remove the need to roll. While most of these aren't viable for your build (e.g. the 10th level Rogue's Reliable talent to set a floor of 10 on all rolls for which you have proficiency, or the second level Divination Wizard's Portent to give you pre-rolls that you can use to replace a bad roll, depending on your luck on the rolls that morning), there is one option that becomes very interesting for you at higher levels (when rolling becomes less and less reliable without lots of powerful magic items): The 8th level Bard spell Glibness.
For one hour, with no concentration required (so you have no limits on what you can cast, and no risk of losing it to damage, and it should last a couple of encounters if you're in the thick of it), you can replace any Charisma check's roll with a 15, raising your minimum roll by +14. With Expertise and Cha 20 alone, from the moment you gain access to Glibness, you'll always succeed on any attempt to use your item for any spell (your minimum roll at level 15 will be a 30). Downside: It does eat your one and only daily 8th level spell slot. But that's not a terrible tradeoff for going from 19-24 Bard spells known to effectively having the entire Bard spell list as your spells known for an hour a day.
