but should I buy the 1000ft of cat8 cable needed to put 2 jacks in every room in my family's house when I have 702 feet left of what used to be 1000ft of cat6a, and the maximum length cable should be less than 100ft?
If we're talking about future proofing, you should run fiber in parallel with all of your cat 6 cable.
From my personal perspective the benefit of cat 8 is not significant enough to warrant buying another spool, but if it's only like 50 bucks in your area by the spool.
Fiber is dirt cheap, terminating it is expensive, you can just run the raw fiber and leave it for the future.