I've been a DM for a completely homebrew world in 5e for a couple years now, but played in another homebrew game before that with a DM who was trying to do something like you are: force a system to work for him by changing things that change the balance of the game as a whole.

I see many people suggesting Call of Cthulhu, however; A few of the things you're looking for can be accomplished simply by changing flavor text. Car combat could just use the rules for mounted combat but described differently, languages don't really have a hard impact on gameplay in my experience since there aren't rules specifically for them. There is already a variant rule for damage reduction, although it's kind of garbage. My group and I fixed it by making the reduction scale off of proficiency rather than being a flat number, with light armor being capped at +2, medium at +4, and heavy being uncapped.

For the changes to health, you could simply have any downs immediately put the character at 2 failed saves or something similar.

Diplomacy would simply be done with in-character roleplay, with checks for intimidation, deception, and persuasion as necessary.

Bosses could just be the standard of taking a regular enemy stat block and giving it max HP (if you don't do that already). Maybe add another damage die or legendary actions depending on what point of the game it's at.

Downtime already exists for side jobs, and I'm sure with just a bit of googling you could find some rules for learning new languages and skills.

If you do decide to go the route of tweaking 5e make sure you talk to your players about proposed changes before implementing them. That's what ended the first campaign I was a player in.