But I cannot access this folder on Android? Otherwise I had used Syncthing.

I use filen actively

But, how to use it as an Alternative to firefox sync??

Hmm,
Will self hosting sync solve the problem??

Btw this metadat collection will happen anyway if you use addons (though not linked to your account)?

So I should use ffsync for now i guess. Is there any info regarding what unencrypted metadata they can link to me i use ff account?

Can I self host it from termux for localhost only?? Without need of domain and other resources??

Is not ffsync encrypted?? I guess the downside is

1. Unencrypted metadata
2. Projected future

But i heard Sync feature encrypts data on client side And it is end to end encrypt

Personally never used WG;
But I had to use I2P + Tor + Firewall on my droid, but as all these require a VPN on non rooted phone, and Android support just one VPN at a time, I am now using Invizible pro which let me do this all from one app and then setup VPN from this app.
Point is find (or create) and app which can access all protocols you are trying to connect.