Hi,
I do not believe that you can obtain an older iMovie version to run on your Mac when you already are running the most current version for Big Sur. However, obtaining an older version likely won't be of help re your import issues. Also, most digital tape recorders are supported even though not displayed on Apple's list of supported cameras.
Importing into iMovie 10 from a digital tape camcorder can be hit or miss. Sometimes it works and sometimes it
doesn't. Most import problems are caused by faulty cables or adapters, or possibly a faulty socket on your Mac. Check out your cables and adapters and make sure that all fittings are tight. To import from a digital tape camera you would need a firewire cable coming out from the camcorder and plugging in to the firewire socket on the Mac. Newer Macs do not have a firewire socket so you would also need whatever adapters are necessary to connect to one of the thunderbolt sockets on the Mac ---Thunderbolt 1, 2, or 3 (USB-c). If the cables you have been using are not Apple cables, you might try swapping them out for Apple cables.
If the camera still is not recognized, try turning it on and playing the tape before beginning the import process. Also, toggle it on and off to see if you can trigger recognition.
You can also try importing into the QuickTimePlayer app on your Mac, instead of directly into iMovie.
Take a look at this thread that discusses importing from a Handy Cam:
Importing mini DV tape from Sony Handy-ca… - Apple Community
-- Rich