SUper frustrated as I write this, mainly because I completely butchered a piece I thought I had on lock.
Im currently playing Baermann's Tarantella, it's a challenge and I've found it mostly enjoyable. However, I find that my ability to play it varies hugely from day to day. It's a very fast and lively piece, so some days my fingers can keep up with the speed and it sounds great, other days my fingers feel like they're jelly and I fumble over every little tricky part. Is it something that gets better with experience? Thanks in advance.

You need a stronger foundation. Determine which bits in particular you stumble over and practice those parts slowly and methodically and consistently even on the days when you're not stumbling.

I feel like this is not actually a case of familiarity, but mechanical skill. I was playing again just then, and I couldnt even do the first four bars correctly whereas I was able to play it through at full speed 2 days ago. The feeling was one where my fingers straight up refused to move as needed at the same speed. Are there any ways to improve finger responsiveness?

I hate this black soundmaking stick sometimes .

Technique is all repetition until you can play all combinations of notes smoothly and rapidly. Then there is a certain mental aspect. I find that my playing improves somewhat if I'm in the right frame of mind--particularly with sight reading. You may be ready for my book mentioned on the forum here. There are a number of very difficult books exploring odd and difficult fingerings--one such series is by Rudolf Jettel. I studied those in the '70s and still review them today.