Most of the classes, races, and backgrounds are forms with various modifiers you can add/remove/tweak in the UI. So those won’t need any real coding unless you’re trying to do something REALLY custom like a new mechanic that doesn’t exist in DND core at all. I think a good example of really custom would be a star shaped spell area of effect. Possible to do, just might need to get a module or code one yourself if no one else has.

For the system rules those are harder to edit and you will need to code for that. If you can make your modifications work in roll20 though I would be surprised if you couldn’t do that in foundry.

Honestly the biggest selling point for me was the plug-ins/modules the community can make. I would take a peek at some of those as they’re such a game changer compared to roll20 and the other vtt’s I’ve used.

I’ve been running games on foundry for 2-3 years and don’t think you’ll have issues with custom classes, races, or backgrounds unless you are doing something really really wild.

The core mechanical changes are going to be harder, but compared to roll20 it wouldn’t even be a competition foundry is just better for customization.

The most annoying thing is probably going to be porting all your homebrew. You can share it between worlds though via compendiums which is a nice timesaver.