I’ve only tried TamperMonkey and Userscripts, the two most common ones. I’m aware of PageExtender and I’m currently downloading Xcode (it’s $5 on the AppStore or free from source) and will try it if I get a chance. I see an issue on their Github page about CSP errors on some sites though too which makes me think this might ultimately be an issue with how Safari handles CSP on Discourse rather than any particular script issue.
It appears this has been an issue for a while, judging by CSP preventing user script on Wanikani Community page in 2019