Trying to update macOS to Catalina 10.15.4 asks to restart again and again
Solution 1:
You need at least 20GB to install Catalina (macOS 10.15). Updating this OS probably requires the same amount of space.
I’m guessing that your constant restarts are due to the installing not having enough free space so it’s swapping info from the disk to memory.
Try to free up as much disk space as you can, the more the better. Once you get to 20GB, you should be ok. This answer is based on my personal experience with updating many OS versions over the years.