Stick with an older system version that supports macOS 10.14 Mojave, if you still have iPhoto Libraries that need migrating to Photos. Before you upgrade to a newer Mac model that no longer supports iPhoto you have to prepare your iPhoto Libraries for the migration to some other photo manager of your choice. It is not just the problem of missing support for iPhoto and Aperture on the newer 64-bit models. Some media formats depending on 32bit frameworks will no longer be supported as well. There may be older videos needing QuickTime player 7 or image formats, that can no longer be read and you need to convert them to a compatible format, while you still can open them.
Get your iPhoto Libraries shipshape and convert all items in them to a format, that can be opened on a newer Mac with macOS 10.15 or later. I have used this list for iMovie to find legacy media that are no longer safe to use: About incompatible media in iMovie for macOS - Apple Support
And I kept one Mac running Mojave, so I can still convert items that I missed, before upgrading to Big Sur. The older legacy formats ar turning up everywhere - in old Keynote or PowerPoint slides, in older iMovie Libraries on backup drives, in older backed up Aperture and iPhoto Libraries.Some of the older iPhoto Libraries are needing the iPhotoLibrary upgrader, that will not even run on Catalina, because it is also a 32-bit application. You will not even be able to open old iPhoto 7 libraries in Photos for Mac. They need to be prepared for the migration on a Mac with macOs 10.14 or older.