I don't think DNSSEC would help in the common case of non-validating stub resolvers querying a public resolver. My understanding is that the DNS query response from a DNSSEC-validating public recursive resolver doesn't contain the information required for the stub client to validate it, only a single AD bit.

Probably not, I can't remember the last time I looked at 'resolvectl' output and saw anything other than "DNSSEC: no" on any system so I assume it mostly just doesn't exist in practice

More practically: do you have DoH enabled? If you're using Chrome, the answer is probably yes.

DNSSEC doesn't help here. It doesn't run between stub resolvers and recursers like 1.1.1.1.