The main issue is that I spend the last 30 minutes trying to softer some edges on a layer that goes on top of another one..

Attempts:
-feather: blurs the entire image
-erode/dilate: someone was smoking crack when creating that one
-spitting into RGBA and blurring A in various ways before joining them again is also not working.

So.. what did I miss?

This is hard to answer as the goal could be many things...

Assuming you have some hard-ish cut-out that you want to soften.

Often this means there is no "data" in the cut-out parts - so things like blur that expand the softness outwards can't work, as there is no idea what the colours should be there.

This often thus requires a pure inward feather. This is what feather can do, the trick is you want to do it only on Alpha.

So, rgba -> rgb/a split. Then feather on a. Then rgb/a -> rgba to combine and re-multiply. If you have a zero-one alpha, you can use a remap to adjust the shape of the fall off to make it less "linear".

Attached shows this in action.

jlait
This is hard to answer as the goal could be many things...

Assuming you have some hard-ish cut-out that you want to soften.

Often this means there is no "data" in the cut-out parts - so things like blur that expand the softness outwards can't work, as there is no idea what the colours should be there.

This often thus requires a pure inward feather. This is what feather can do, the trick is you want to do it only on Alpha.

So, rgba -> rgb/a split. Then feather on a. Then rgb/a -> rgba to combine and re-multiply. If you have a zero-one alpha, you can use a remap to adjust the shape of the fall off to make it less "linear".

Attached shows this in action.

Ahh I missed the pre-multiply alpha after rejoining the channels with the feathered alpha, thx!

Just a reminder: when used as a height map, the "feather" method still looks quite poor—even in circle mode—and produces noticeable stepping artifacts. The screenshot shows "feather" on the left and the same shape rendered via SDF on the right; the difference is obvious. Perhaps "feather" is faster? Honestly, I’m not sure where it’s actually meant to be used. It did work well for square corners once, but for smooth curves, it’s a definite no-go.