Depending on the game and comfort with bash scripting you can roll your own mod managers. I don’t really play Minecraft anymore, but if I did it would be heavily modded. In an effort to avoid installing a client/launcher beyond the one I already use I just keep folders for mod lists and configs, and then have bash scripts with aliases to do all the necessary file moving to swap between mod packs.

This doesn’t really work for most other games, but for things that run natively on Linux can usually do the trick.

For things running through proton it’s a bit more involved, but I also found a lot of satisfaction in figuring out how to manually install mods within the proton prefix. Used to have to do that a lot to mod Skyrim when it first came out and I got it running through wine on a school issued MacBook.