GSA PS has already a lot built in sources, but there never can be too many (as long as they working). At https://www.blackhatworld.com/seo/proxie-scraping-links.464079/page-13#post-9381358
the super-hero public provider Proxy Go (disclaimer: I was customer for years for his SB proxies a longer time ago) provided a huge list with scraped proxy-sources.
I think you cover the most of the resources, but if you want to look over the list, I've downloaded the list, deduped it and then verified that the URLs are working. From a super huge list, still a huge of 15K URLs remains. Around 1.5K of it were already in GSA PS, the others can't be directly imported into GSA PS (what format do they need), but it's maybe helpful for you @Sven
or someone else who does the next steps.
Download is available for 30 days: https://ufile.io/88d10
Cheers and have a good start into the week