Alright, now I'm getting somewhere. Thank you! I inserted the SD card and did the reboot update again. This time it boots into LibreElec. It's now stuck (with the rotating dash animation) at the "Resizing /storage" step since more than two minutes.
I think something went wrong.
I'm using a fast Samsung Evo+ 64GB MicroSDXC card (with adapter). Is it OK to use SDXC cards or should I use smaller SDHC cards?
Is it safe to turn off the device?
Never mind! I waited a little longer and finally it proceeded past this step! Fantastic.
Next problem: The remote is not working at all. I guess I have to find a working remote.conf for the V88 Pro somewhere?