I'm a very stubborn person, so I just assume that writer's block is something that doesn't exist (or only happens to other people, not me), under the assumption that the only people who get writer's block are people who know what it is. This isn't to say there's not times I'm staring at a screen thinking "what happens here and how do I phrase it, ugh, I have literally everything in this arc written out except this one pivotal scene", it's just that I've found it more helpful to assume it's a matter of my own effort rather than a demon visiting me, if that makes sense.
You might find it helpful to write scenes out-of-order, whether it's just writing the final scene of a chapter first or even developments you're planning on happening 30 chapters away; and as for writing quality, if you're consistently improving, you're probably going to look back on your work from 6 months ago & think "god, how could I ever write that?" There's going to be ebbs & flows where you even get an idea that's only 'decent', but you just gotta work with it and try to make it as good as you can, accepting that especially early in your development (it took >1,000,000 words/2yrs for me, admittedly) there's going to be high & low points. The important thing is that you feel you're -generally- improving over time, not necessarily that every chapter must be better than the last.
I've definitely done what you mention before too - writing a short novel or story even in-between working on my main squeeze, though that was back before I was posting it online. Fanfic can be fun for that, since it's necessarily less effort to work with existing characters & setting, and once you finish up (or even work on both!) you'll likely have improved and be raring to get back to the main one.
It took me about 1 million words written total to get to the point where I felt okay showing it to other people, so you might as well get there however you can, accept that you can later edit it, and that just getting a first draft out helps - it's just a fun hobby at the end of the day. It's a little more complicated since you're posting it & have active fans, but I hope that helps some.