My PC is using a musket based on the standard rules for firearms in the DMG (p. 267), which includes reloading after every shot.
A limited number of shots can be made with a weapon that has the reload property. A character must then reload it using an action or a bonus action (the character's choice).
If I have an extra attack, I can't use my bonus action between the first attack and second attack to reload my musket, right? Word of God on the matter seems quite clear - per a tweet by Jeremy Crawford:
No general rule allows you to insert a bonus action between attacks in a single action. You can interrupt a multiple-attack action with a bonus action/reaction only if the trigger of the bonus action/reaction is an attack, rather than the action.
As there is nothing in the rules as they are written to allow me to reload my musket to use it for my extra attack, would asking my DM if we can brew a custom feat allowing me to reload between attacks (and sacrificing my bonus action to do so) make my character overpowered?
This doesn't seem too extreme, given that it is less potent than the Crossbow Expert feat ("you ignore the loading property of crossbows with which you are proficient"), which compensates for the musket being more powerful.
For clarity, I am aware that my character would be starting the next round of combat with an unloaded weapon.