Try a core update to v23.1.3 now. It has a new "Edit..." context menu for the folder theme image selector that brings up a larger version now.
Thanks for your effort!
This is better, but that is not what I had in mind. You can now only shift the cropping area up or down, see attached screen shot.
I proposed above to combine the edit en crop focus function in a new graphical window like you showed in the new Edit function, but it should be possible to drag the 4 corners of the cropping area to exactly the part of the picture you like to use as theme image.
For an album with folders, it should of course be possible to save the result per folder.
I see now that a cropping size of 120x60 gives the same result as 1200x600, see enclosed screen shots. I think that is weird.