Same here - the bead always gets cleaned up. As do any large deposits from pooled sealant that might add a little imbalance. Really what I find myself doing is spend 5-10 minutes per tire, and then just add fresh sealant.
With that said, I do think having some of the old stuff can be a benefit in terms of filling smaller holes quicker and losing less pressure. But that only really matters on the smaller ones that are going to close up pretty quickly anyways, and I don’t think I’m ever using old tires for an important event anyways…
I think it depends on the discipline. For a bike only used on road, I’d only swap tires around for races, otherwise just ride them until they wear out. For MTB/Gravel, I don’t think I’ve ever worn out a set of tires in a single season. But it’s also rare that a set of tires stay on the bike more than a month or 2 straight. And when racing a lot, tires often get swapped constantly depending on course. And I typically try to save race tires for race days, prefer to have fresh rubber on when racing if possible.