I’ve now unwrapped each toe separately after having added a seam to each toe. The “UV Map Islands” are almost overlapping perfectly and I’ll use snapping to make it perfect. Now, there’s no texture stretching on the toes:

I don’t understand the sense of a “Join as UVs” function if it’s so sensitive and easily produces messy results. Then it’s easier and faster to do it manually than wasting a lot of time with an unnecessary cleanup.