@ari I have not tried Fluxer, but I have seen that it blatantly clones some of the worst features from Discord.
If you have not read https://blog.fluxer.app/how-i-built-fluxer-a-discord-like-chat-app/ already, I suggest you do.
The article addresses the vibe-coding concerns (which say it's not "vibe-coded" but LLMs are being used, so... the concerns are still valid)
Regarding Matrix, client feature support is indeed the major issue. The 2 clients I see recommended most often for Discord users are Cinny and Commet. Both of these clients are... fine for casual use. They are both missing major features and for various reasons I can't recommend either as a perfect replacement.