Yesterday nothing worked. It just went through the steps but never updated. So today I thought I try again and when booting it went to filevault and then updated as it should have. I know I didn't do anything differently but I'll take the update.
Updated to 10.15.5 from 10.15.2. Three reboots selecting 'prebooter install' from Clover each time. I'd also updated Clover to version 5119 before running the update (using Clover Install/Update Clover to download the latest version). And I used the Clover Tool to update the Kexts identified as being out of date (using Kext Installer and then installing them to the 10.15 directory). No issues. It pauses for a bit in each of the install loops, particularly the first one, bit of patience and it went perfectly. Everything working.
I want to warn against one problem. I have no idea if it's because of Catalina or Clover, but it started with Catalina's arrival. I noticed that Catalina / Clover overwrites the BIOS.
Half a year ago, it happened to me that the boot disk in my BIOS changed so that it could not be changed back normally, but it was necessary to set no disk, then set another disk, and then the desired disk. Now with the new version 10.15.5 it happened to me that when I used the sleep mode, it could not wake up and after hard shutting down and turning on the computer, the BIOS beeped the HW error and the computer did not boot until the third reset. So I stopped using sleep mode (it worked well before) and started shutting down the computer.
Then came a quick fix 10.15.5 I tried sleep again. It didn't start after waking up. After the reset, the initial screen of the board was displayed and then restarted. When I entered the BIOS, it worked normally. I restored the profile. Attempting to boot from disk restarted again. I suspected it would be a strangely rewritten BIOS again, so I loaded the default settings, but that didn't help. I also re-flash bios. It didn't help either. I tried to remove one or the other memory, I tried to pull out the graphics and boot from the processor graphics, I tried to disconnect everything from the drivers and boot from USB, it still didn't work.
I already thought it probably wouldn't be the BIOS but the ATX power supply died (if it takes more weight to load, it turns it off). I bought a new power source but it didn't help. So I accepted that the motherboard had died and that I would have to build a new computer. And how much money will be in hell again if this one still works satisfactorily.
Finaly I tried flash older bios and it ran!
Eventually I re-flash the latest BIOS and it works. Apparently OSX rewrote something in places in the BIOS where the new BIOS did not rewrite anything and fortunately the older one did.
In the end, I'm fine again, but I'm writing this warning. For me, it was on the Gigabyte Z170x Gammin3 (EU) board, but I do not rule out that it may be different. Or other strange behaviors may occur. I think Catalina does it (paranoidly, I mean intentionally - as a fight against hackintoshers), given the correlation with these updates (but it certainly doesn't have to be causality).
(I have current kexts and clover)
Edit: This morning, when I turned on the computer, the bios was rewritten again, even though I didn't put the computer to sleep, but just turned it off. Fortunately, only the parameters, so it was enough to upload the profile back. (from disk - saved profile in bios disappeared) Clock is current - it is not a discharged battery. In addition, I measured the battery yesterday.
10.15.5 is certainly messing with my BIOS, too. Since updating to 10.15.5, every time I clone with Carbon Copy Cloner, a spurious macOS drive is created, and my BIOS sets it as my boot drive. Of course, it can’t boot so I have to go into my BIOS, and reset it to my real UEFI boot drive. I can delete this “drive” in Clover’s (r5118) shell, but, the next time I clone to either of my backup drives, I’m right back where I started. Even with two updates to CCC, it still does this. I’ve used CCC for seven years and haven’t had this happen. I reported this issue to Mike Bombich plus another issue created by 10.15.5 that he did resolve, but it’s certainly something Apple has done.