Have TM treat a clean install as a new machine (keep the old backup)
You've already done it. By erasing the disk you changed its UUID, so TM already considers the new disk completely unrelated to the old one.

The only problem you may run into is that if you ever do a full-disk restore from one of your old backups, TM will patch things up so that the restored disk (which now has yet a third UUID) is considered the one that all the old backups were from. It will now be the new backups that it won't ever erase.

Which come to think of it is probably exactly what you want.

In case that's confusing, let's take a specific example. You have a disk with UUID#1, and in March you begin letting let TM make a bunch of snapshots with it. TM remembers that they're snapshots of UUID#1. Then in June you erase the disk, so that it now has UUID#2 and let TM start taking snapshots of it. TM regards your snapshots from June onward to be snapshots of a different disk from a different computer. It won't touch the March-April snapshots.

Then in August you do a full-disk restore from one of your April snapshots. Part of a full-disk restore is to erase the disk, so it now has UUID#3. But at the end of the restore, TM will patch up the backup so that the March-April snapshots now appear to have been of the disk with UUID#3. The May-June snapshots, of the disk with UUID#2, are now the protected ones that TM will not touch.

Personally, though, I'd let TM start backing up to a new disk. That makes it easier for you to keep track of which backup is of which disk, and reduces the risk that you'll lose the old backups to a drive failure. Drive failure seems to be the biggest threat to a TM backup. If you aren't using the old backups, you don't need to have them mounted all the time.