Found this as I was struggling to decide what to do with my Aperture as I finally upgraded from Mojave.
The answer: Retroactive! Turns out someone made a hack called Retroactive to allow Aperture (and iTunes as well if you miss that bloatware) to work on the latest MacOS.
So no need to migrate for the time being. The method of support doesn't look too heinous, I'm surprised Apple just didn't do it themselves.
It's been around for awhile now I'm surprised I didn't hear about it. I was trying to migrate to Lightroom during the Pandemic lockdown but it was sooooooo sloowwwwwwww and would sometimes crash that I gave up. (I had started to split my library into pieces and them importing piece by piece but ran out of energy for that too).
Retroactive has been discontinued. You should transition from Retroactive to supported apps such as Music, iTunes for Windows, iMazing, Photos, Darktable, Lightroom Classic, and DaVinci Resolve. - ...
github.com