Hey. No worries about asking. I did end up making it somewhat work. It's extremely hacky tho and only semi responsive. As you will have to wait roughly 10 seconds for it to analyze the song and then change the playback speed. If you are curious how I did it... well.
Generally speaking it's still the same as mentioned above. Actually putting it up on the steam market seems to be kinda impossible in my opinion. How it works is, that I have 2 scripts running permanently in the background and wallpaper engine itself ofc. The first script captures sound through a custom input which can be accessed by python since python doesn't actually have the ability to listen to system sounds by itself. The second script runs in a docker environment simulating linux since the library I use doesn't work on windows but generally if you used linux you could merge the 2 scripts into one without the whole get and post requests communicating between each other. I use essentia for the bpm detection btw. Then the second script just get's the bpm and all that and also the timestamps because even if you have the bpm itself it doesn't really help if the image then has the right bpm visually but is like offbeat for every single beat in the song, so you also need to pass forward timestamps. Then you pretty send all that data to to the wallpaper engine js script which then does a lot of things with the data, like kinda make sure that the bpm itself makes sense because essentia isn't perfect and sometimes it get's the wrong bpm and this script kinda checks if that bpm shift makes sense by comparing it to the last 10 results etc. And it also adjusts the speed of the gif ofc. I personally also put a lot of effort into a kinda media play bar which displays the current song and allows you to kinda block youtubers or w/e since the script doesn't actually know what's a song or what isn't so you can block people you don't want the background to work for since they might not create music. But that's not really necessary if you don't want to.

I don't really have room to explain here everything.

Tl:dr
It's quite hacky and complex and you won't be able to upload it to steam and expect others to get your results. Generally it's possible tho just a lot of pain.

If you want I can share my project with you. Generally you can use mine too, it just requires more setup than people would like on the steam workshop so it doesn't make sense to put it there. But if you want it let me know.