I saw one other person mention RetroAchevements and I second their love for adding it to older games-- in these cases, though, they’re a passion projects by fans and while they maintain a leaderboard, the main thing to be is it legitimizes some retro gaming by disabling save states and cheats while still having sets for modded games. I think it sets a good standard for what the industry should be doing.

For instance, it checks those boxes; it’s opt-in by default and there are privacy options, although hardcore mode requires rich presence for enforcement of the rules. There’s still the softcore option though. The sets have rules about what you can make, leaving grindy and multiplayer stuff as subsets. And again, modded/hacked roms get support, since all it takes is a user wanting to develop it (except Pokemon Clover, that’s banned for pretty good reasons lol)

At this point, it feels more legit than Steam, Xbox or PlayStation achievements. If the industry adopted a similar model, most of your concerns would be addressed although it’s likely impossible since developers often have to write achievement code that fits all platforms.

(Oh, and if you can’t tell, my answers to the questions, mod use makes it irrelevant on PC and permanence does matter to me, although not for any good reason: RA badges are nice to look at. Lol)