The Man with Two Guns is God
You have discovered the coolest fighting style in the world. You are proficient with gunsmithing tools and with all firearms. Whenever you take a long rest , choose two firearms in your possession to maintain. Those weapons suffer no mishap chance for the next day.
If you are wielding two weapons, when you use the Attack action and attack with a one-handed weapon, you can use a bonus action to attack with a second weapon if that weapon is a one-handed firearm. You can reload firearms even if you have a weapon in both hands. If you are holding two firearms, you can reload both with a single action or bonus action.
After you hit or miss with a firearm, your target cannot make opportunity attacks until the start of your next turn.
You can draw and stow one-handed firearms without using your ‘object interaction’ for the turn.
Additionally, during a short rest you can tinker on a firearm, using raw materials (i.e., directly spending money, without needing to buy accessories) to apply any of the firearm upgrades. You can transfer an upgrade between weapons without needing to expend any materials.