It’s not meant to be selfhosted.

https://github.com/revoltchat/self-hosted

it’s not federated

I’ll agree on that one. Though the FAQ says that it is not on their future roadmap, but is possible. https://developers.revolt.chat/faq.html

The developer seems very shady

Why?

https://revolt.chat/

FOSS alternative that looks and feels the same.

Your best bet would be to shred the data multiple times (for example with the shred command) and then break the card physically. But shredding takes time so I guess that’s not very applicable to your case.

If you have a lighter, you also can try to melt the SD card’s insides. That should be impossible to recover.

In any case, you should keep it encrypted all the time and only decrypt it on the fly. For example with LUKS2.