You can decide to move to a Pro Tools subscription or purchase a expired license renewal... the later is only available from Avid dealers not from Avid directly. e.g.
That would be something like
https://www.sweetwater.com/store/det...e-expired-plan
But even better get on the phone and call a Sweetwater rep and make sure that is exactly what you need.
I do not follow why you are asking about "going off sync" there is no reason to think interface sync will work any different with later versions of Pro Tools. Were you having problems before? I just don't even understand why you are going there. You do need to make sure you have drivers for all third party interfaces on the macOS version your iMac is running... like the 18i20... make sure you install the correct drivers for the generation of 18i20 you have. And Forusite have good documentation and a compatibility checker on their website.