I highly recommend using Unexpected Keyboard along with termux (a recommendation I myself almost certainly got from HN).
On the transfer, here is what I could dig up:
The github issue about it was deleted, but archive.org has copies: https://web.archive.org/web/20251215062049/https://github.co...
HN discussion of same (with another link to the syncthing forum): >>46184730
Lobsters discussion: https://lobste.rs/s/urbcpw/potential_security_breach_syncthi...
(and here is the announcement that the official android syncthing app was being discontinued: https://forum.syncthing.net/t/discontinuing-syncthing-androi...)
No shortage of reading if you have the time! I'm quite happy to be running just the "standard" package (although, yeah, I should've pointed out that I don't run in continuously on my phone...)