Thanks! It looks indeed like libXISF is just missing.
sudo apt list libXISF doesn't return anything which I think makes sense because I bypassed apt by making and installing manually.
It shoud have been build and installed by the script, as it's not part of the usual repos of Raspberry Pi OS. Also I can't install it via apt because it's not in my repos.

But it looks like the the libraries are there at /usr/local/lib/aarch64-linux-gnu/ which I think is an odd location but that's where they were put during script processing.
I guess that's not the location KStars/Ekos expects or maybe some linking is missing.

Maaayyyybe I just have to switch to Ubuntu to make everything easier but I kind of like Raspberry Pi OS.

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