I have an old Aperture photo library and I am now planning to finally upgrade from Mojave to Catalina on that system. I know I need to migrate my photo library but I'd like to know in what order I should do that. Upgrade to Catalina and then use the version of Photos to migrate my Aperture library or migrate first and then upgrade to Cataline. The former would use a newer version of Photos with possibly(?) a few migration bugs ironed out, the latter clearly has the advantage to still be able to open my library with Aperture until I'm sure I'm happy with the migration. So, any other thoughts, things I am missing?
I would be sure you delay losing Aperture until you’re sure you have a good import. I haven’t needed to load Aperture myself, so it might be a moot point, but better safe than sorry would be my advice. Go to Mojave and get an excellent backup you know you could restore in bootable format if you have professional photos, pro level metadata and edits / albums or can’t reimport them later.
You surely can reimport the raw images, so if that’s an option, going straight to Catalina might be faster for you with not much extra risk.
When I moved all in on Photos, I purchased Power Photos to help split libraries since I knew all my photos were 250 GB and I didn't want to pay for more than 200 GB of iCloud storage and I'm all-in on cloud library with a local backup. I've got about three Photos libraries and one has my main personal work that's in iCloud.
It has served me very well and I'm still barely making it in 200 GB. I'll probably splurge for Photo Mechanic when I next rework things. Capture One also gets super high marks. I use both of these professionally but for personal use Apple and Fat Cat Software have served me well enough.