I would make packets over the MTU truncate (with a flag indicating the truncation), adjusting the IP checksum (the same way it's currently adjusted for changes

I would make packets over the MTU truncate (with a flag indicating the truncation), adjusting the IP checksum (the same way it's currently adjusted for changes in other fields like the TTL) to match, instead of fragmenting or dropping. That would avoid all the issues with PMTU blackholes or non-initial fragments.