After all this is settled, you need to strongly consider a round of MAJOR housecleaning getting rid of old MacOS versions and repartitioning and reformatting both of your drives. Your drives are becoming so fractured the partitions may become unusably small and unlike APFS volumes removing a partition may not be possible or it may destroy data in other partitions. You really need to simplify.
Your idea of moving partitions with antique MacOS versions to thumb drives may be the best solution to your current situation after all! (Then you can throw them into a desk drawer and forget them.😉)
If we knew what it was we were doing, it wouldn't be called research, would it?
