com.apple.photos.VideoConversionService

To repair the Photos Library you do not need to restart the computer.

Repair your library in Photos on Mac - Apple Support

Quick summary: Close Photos then option-command-double click on the Photos icon.

You should try making a new user and see if the same problem recurs with it

Also, use another library if you have one, or make a small test Library with a few pictures in it and see if the same thing happens. You can create a new Library by closing Photos and then option-clicking on the Photos icon.

The "hold D down after restart" thing is about resetting the PRAM.

Mac startup key combinations - Apple Support

It may be worthwhile to start in Safe Mode, but you do need a wired keyboard for that. There are some pretty cheap USB keyboards-- like < $15, but I don't know how well they work on a Mac. You could get one on Amazon and send it back if it doesn't work, then try another.

OldManMikeH wrote: ... will be adding a Hd external for second lib in about 7 days. I use SSD for Time Machine.

Great. Be careful with the new hard drive-- it probably needs to be re-formatted. To avoid damaging the Photos Library an external drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. Additionally, the drive can not have had Time Machine on it since it was formatted. There have been so many problems with using incompatible drives that the newest macOSs won't even allow a Library on a non-Mac formatted drive to open, since there is a chance of damaging the Photos database. See this:

Move your Photos library to save space on your Mac - Apple Support

You might also want to check the permissions for your new drive, if you're sure it's properly formatted. This not usually a problem, but it sometimes comes up.

Click on the drive icon, and open the Info window with command-i. Go to the bottom:

If “Ignore Ownership” is there, then it should be checked.

Good. Keep at it!