I used the legacy version of Final Cut Pro from almost the beginning up through version 6.06. It got more and more difficult to migrate this to each new version of MacOS. Trying to remember, but I think I upgraded from version 5 to 6 because there were a lot of new features. When version 7 came along (the last one) there weren't many changes and I didn't upgrade (it was very expensive back then).
I stuck with version 6 for many years and finally stopped at (low) Sierra around 2016. IIRC, the problem was that, while FCP itself was a universal app, the INSTALLER was not and needed the Classic environment to run. Sorry, has been a long time, but the bottom line was you could move an existing installation to a new Mac with the Migration Assistant but couldn't install from the original disks. Back then, there was discussion of extracting files from the disks, etc. but that was complicated and not sure if it worked. But even if you were able to install, the problem was that it crashed a lot on newer operating systems, which was just frustrating.
But this was FCP 6 and 7, FCP 5 would no doubt be even more complicated. I made the transition to the new Final Cut Pro on Catalina. It is a COMPLETELY different program from the original Final Cut Pro. Seriously, the name is the only thing it has in common. I got used to it and like it very much now, but it took awhile to get up to speed. If you do switch to the new version, be prepared to start from scratch.
There is also no compatibility with old Final Cut Pro projects - you cannot import them. Somebody made an app to help with this, and it does, but not much. It is basically like starting all over with an old project. I still have my old 2013 MacBook Air with FCP 6 for the times I need an old project - which is hardly ever these days.