There's been a bit of progress! Here's a low-quality soft shadow, showing off no z-fighting and no blur over top of letters in the single-pass shader! The shader has code that expands the size within the verts without altering the text UV coords to allow room for shadows and it's honestly really crazy that this is even possible.
Progress got halted again because the unity asset store caused me some problems a few weeks back, I wasn't even sure if I'd be able to give customers asset updates anymore so it was a bit demotivational. Hopefully we'll get back to work on the shaders soon! (It's actually my shader friend's birthday today so... maybe not immediately haha)
Also, I thought it was 100% broken, but as it turns out that Super Text Mesh used with Unity UI should still support the current outline/dropshadow shaders. A friend was using it in a URP game the other day with a screen-space canvas, but I'm curious if it'll work with a world-space canvas, too. Try giving that a shot!