Bitsearch and Solidtorrents’ Login and Sign Up buttons don’t work at all for me, even with extensions disabled (Firefox, Chromium)

I don't think you require an account to upload/add a torrent hash there, give it a go without registering.

TPB either loads the takedown page (brazilian one), or refuses to load (VPN), otherwise I’d upload there, I have an old account.

See the TPB official domains in their forums https://pirates-forum.org/status

For blocked websites you can try changing your system's DNS, or use a VPN, or use a proxy, or use Tor Browser.

Note that you do need to register/post in the TPB forums to request to register/upload at the main TPB site.