I still highly recommend Mullvad, they are generally an awesome company and havent had many issues with them beyond a few random sites blocking them which is to be expected.
Mullvad used to be the no-brainer until they stopped supporting port forwarding. I kinda need it for seeding. ProtonVPN seems like a good idea for now.
I find that weird that so many struggle without port forwarding… my seeding is going just fine without port forwarding.
It works fine in public trackers due to the abundance of participants. In private trackers, it matters much more.
What is the difference exactly? Can’t tell if I’m missing something fundamental or what. In my head there’s no reason you’d run into issues

There’s no technical difference, but there are differences in practice.

First of all, in order to transfer any content, one of the participants need an open port. In case of public trackers, there are usually a lot of users, so someone will probably have a port open that you can connect to. Also, many don’t even realize that they have open ports, since routers can open them on demand by uPnP. For private trackers, since the userpool is much smaller, these matter more.

Also, for public trackers, you don’t really HAVE TO seed and maintain a good ratio. It’s the right thing to do, but is by no means a necessity. For private trackers, on the other hand, your account will be suspended in most cases if your ratio goes too low.

I ask because I am on several private trackers and haven’t had issues downloading or seeding using mullvad. By what you explained, it shouldn’t be working…
I might be wrong, but I think port forwarding is working for already set up ones till 1st July. Or maybe you have a leaking port through uPnP? Again, just possibilities, not sure.