I don't know enough about Proton or NAT-PMP router support to comment. Hopefully someone else can. What I can say is that, based on my knowledge, PF does not directly influence your ability to download, but like I said, it indirectly can influence your ability to download if the original uploader/seeder is gone and no one else in the swarm has PF or they limit their uploads (like you do, which is understandable). I don't have PF with Mullvad & I still obtain new or recent releases without a problem. What's "recent" really isn't definable, it more depends on the file popularity and how many seeders are still supporting the file that I can connect to.

Example: within a month or so after Mullvad dropped PF I was trying to DL a very popular show (over 8.0 on IMDB & over 80% on TMDB) that had been out less than a year. I tried all variations of 1080p, 720p, x264 & x265. I couldn't complete the entire season from any one source. I was using a few different websites to find access to the files, in addition to using the built-in qBit search function (plugins, including having the Jacket plugin setup), and automatically adding trackers to each torrent that I had copied from "ngosang" and "newtrackon" into the qBit "BitTorrent" section of "options." I never ran into this big of a problem before on something that was still fairly new. I knew I had to choose AirVPN, ProtonVPN, switch to private trackers, or try Usenet. I wasn't interested in private trackers (just seemed like work, to get in with good private tracker/s, having to maintain a good upload ratio, even when others may not want the files I have, and I'm an adult now, with some disposable income, so I'd rather just pay a little, get what I want, and not worry about playing nice with others, although I recognize the importance of it and always supported it in the past). Within an hour of getting Usenet, I had all my files, in the format I wanted.

With Usenet a VPN is not needed, because SABnzbd (kind of like qBit, it DLs the files) has an SSL connection enabled (Secure Sockets Layer, encryption-based internet security protocol). Then you need an indexer (kind of like a phone book or a geo-locator, sort of telling you where the files are located, but the indexer does not provide access to the files themselves) and a provider (a service that actually provides access to that file). You can even automate everything by installing the "arrs" (Prowlarr, Radarr, Sonarr, Lidarr, etc. Do a search for "servarr" and you'll see everything available there.) Link: https://wiki.servarr.com/

There are many indexers, some better than others. Having just one isn't good enough (because their coverage is all a little different than others in regards to what they track). Some you can have limited access to for free. I have access to DrunkenSlug (DS), NZBGeek (Geek), NZBFinder (Finder), NZBPlanet (Planet), NZB.Su (Su), and NinjaCentral (Ninja). Some of these I pay for, around $5 - $10 per year, and others I just use for free. If I had a more limited budget, I'd probably just pay for DS & Geek, or use them & others for free if you don't search and DL much.

There are also many providers, on the same or different "backbones," & some are much better than others. If you want some older files, along with new releases, you need someone with a longer "retention" policy. I went with Eweka, a very popular provider. They have been fantastic for me. An annual unlimited download account cost me $54.35 (with a foreign transaction fee), or $4.53/month. If you only need new releases, then a provider with a limited retention would be fine. I also see many people use a combination deal with Frugal Usenet (unlimited annual access, but limited retention) and it also has a 300 GB "block" with UsenetFarm (higher or longer retention period), from a company called "Blocknews" for $40/Year. Blocks are specific sizes of data access (300GB, 500GB, 1TB, 2TB, etc.) that can be purchased, and generally do not expire. The idea is, most of your recent files come from Frugal, and any older files not retained there should be available on UsenetFarm. I would not buy blocks from other backbones unless you absolutely can not obtain a file from your current provider, and then you want to make sure the block is coming from a different backbone. In total, the annual cost of utilizing Usenet is comparable to many VPNs. However, I continue to use Mullvad (so far) simply for the privacy it provides me, and with a voucher I bought off Amazon, the price is discounted. I got a 6-month voucher for $23.80 almost a year ago on a lightening deal, or $3.97/mo.