Let's Encrypt is 10 years old now

I really wish they would finally branch out and offer S/MIME certificates. Good email clients support them out of the box, it's just a PITA to get them if you don't want to order 100 at a time or something equally ridiculous for SME/individuals.

Would frequent rotation be reasonable for S/MIME certs though?

There's nothing specifically that says S/MIME certs would need to have the same 90-day expiration date, but even if they did, I'm making a basic assumption that if there were a standardised, free API to issue S/MIME certs, major email clients would build-in a client to request a certificate - heck it might even prompt major email providers to offer their own solutions for certs, to compete with alternatives that supported using LE certs.

Once per year or less. Remember to decrypt messages, you need to keep your old certificates/keys around. You can request a new certificate with the same key but i'm not sure that's a good safety practice.