Authy (the app) does support generic TOTP which as you mentioned, so do hundreds of others. Unfortunately, the authy app (and some well meaning but not so well versed companies) opt to use Authy's proprietary OTP which isnt compatible with other clients.

Why not only syncthing? That's what I've been doing to sync across devices (Android, Linux and Mac).

Mostly for legacy reasons... my KeepassXC Android ssh setup predates setting up syncthing. I suppose I could change it to syncthing but it works well as is and there's no real reason to.