Get a VPN with port forwarding. Problem solved.
Is my VPS provider going to nuke me if I dare use my VPS as my VPN, though? Ionos, if it's relevant...
That probably depends on your VPS provider. Some providers have bandwidth or application limits. Hetzner for example doesn't allow people to host or forward traffic from media (Plex, Jellyfin) servers (iirc). You'll just have to log in to your dashboard or browse their rules for that info.
Mostly unrelated to your question and post but you could also deploy Pangolin on your VPS and hole-punch through your CGNAT using one of their Newt tunnels. This is what I do to forward my Plex server to my friends and family outside my home, among other things.
In qBittorrent what is the Status of DHT, PeX, LSD when you click on the torrent and click its Trackers tab?
I’m on CGNAT
Your own torrent peer is not and cannot be connectable (port forwarded), unfortunately.
Not sure if you'll be able to get much improvement, it'll always be slow or impossible depending on the amount of connectable peers in the torrent swarm. But you should be seeing okay-ish speeds if the torrent you're trying to download has tons of connectable peers. Does the torrent you're downloading have a lot of seeds/leeches on it?
PS - If you're saying everything usually works fine then something changed, could try restarting your internet router just to rule that out.
Update: I was somehow able to torrent a different one with aria2c, and it went at full speed, so I'm very sure that I'm NOT being blocked by my ISP...
Sounds like ISP throttling. Are you using a von?
I'm pretty sure it's not ISP throttling but more some weird BitTorrent issue; how was aria2c able to saturate my connection while BitTorrent couldn't move a single byte to save its life?