There is no problem
Seriously.
You're ultimately concerned that the DM may take steps to mitigate the effectiveness of a Feat they created for you. But that is precisely what a DM does; it is their job to ensure that Characters and Players are challenged. It would be absolutely absurd for any Player to expect that they become god-like merely because they are powerful. As Characters grow in power, so too do the threats they face.
Tanks aren't just HP Pools
Additionally, it sounds like you (along with most 5e D&D Players) have a vast misconception of what a Tank is. Having high Hit Points makes you durable; that's it. Unless your fights take place within 5-Ft wide corridors, you're just a durable warrior.
A Tank must be able to control their opponents' movement or actions, or be able to incite their enemies to focus on them. If you have crazy HP, why would they keep attacking you? Any intelligent enemy is going to focus on the enemies doing high Damage; not even a buffoon is going to attack the highly durable melee combatant when they can reach the squishy backline no problem. Enemies aren't automatons who must attack the closest thing; that's what the Confusion Spell does.
If you aren't Grappling your opponents, using Sentinel Feat with a Reach Weapon to keep them from moving past you, or dealing immense amounts of damage compared to your allies...you're just a durable DPS. Enemies are not obliged to let a Character maximize their potential (in fact, that's literally opposite of how they should behave), and a good DM shouldn't let you effectively Tank just because you want to be one.
An IRL Tank is focused on in combat because their offensive power is too great to ignore, not because they are hard to kill. If one doesn't have the firepower to deal with that...they ignore it and go for the squishy infantry units. They don't continue engaging something they can't effectively destroy.
HP is not blood
Importantly, HP is not literal blows. It is an abstract concept, representing such things as Will To Live. Does your Will To Live reduce when someone punches you?
Taking Damage means you get closer to death; that's it. A Hit doesn't have to draw blood, and in fact plenty shouldn't. Someone with 100 HP can't literally take a dozen arrows to their face. It's just that they can mitigate blows, ignore the impact of morale, and literally fight on where another would have given in to mortality.
Take the Werewolf, also immune to non-Magic Attacks. This doesn't mean you can't stab them; it is just that they just won't die. They may get mangled and bloodied, with bones exposed, but because they haven't taken Magical Damage, they keep coming like nothing is wrong.
It would be quite a silly idea that arrows and blades just bounce off their fur.
So by the same token, Enemies shouldn't necessarily notice they're not dealing Damage immediately. That should only become apparent as you continue fighting, and they are confident you should be dead by now.
Magic is as Rare or Common as a DM likes
And lastly, an obligatory: all these posts about how it is unfeasible to give Magic Items so easily have clearly never played anything besides 5e D&D. Previous editions had every item slot filled before you hit 5th Level.
Saying it is unfeasible or unrealistic for most Settings to do such is absolutely facetious; most Settings do have that ability, and only 5e neutered it. The poster-child for 5e, Forgotten Realms, has powerful Casters in most towns, and literally dozens of top-tier Casters (Szass Tam, Khelben Blackstaff, Elminster, Mystra's Chosen, Manshoon, etc.)