to makes matter more interesting — some nodes are on fiber, others on LTE/5G
Oh boy. Then yeah WG MTU is actually too high for the LTE in most cases. Typically LTE has been ~1430 MTU for me in US. Mikrotik defaults LTE interface to 1500, but MBIM reports lower at least for me BUT others report higher MTU (and it would be carrier specific so this make sense), I'd look at the logs for the message described here: see
viewtopic.php?t=171061.
If one presumes fragmentation is problem... I'm guessing the LTE MTU likely what all the IPIP/WG be based on. And you likely don't need the MSS clamping. You'd be given up some space over the fiber. Then again stuff like ZeroTier uses 2800 MTU, presumable to just accept fragmentation but do it big. So maybe I worry too much about the MTUs.
But it actually hard to test this since various things will remember the MTU in one way or another. So restarting browser/etc, clearing connections on router, before a test may be a good idea. But tool sniffer will show the packet size, so you can see what's happening. If you use /tool/ping with "don't fragment" option with various packet size and your far end IPs, you can generally figure out the MTU that way too.
If I had to guess, WG should be 1370, IPIP should be 1350. I'd leave MSS clamping turned off if you lower the MTU. If your LTE MTU >1420, that could be higher. But just a guess (and using Mikroitk rounding so they end in 0)