ISRG (the charity behind Let's Encrypt) has been pretty clear in the past that they aren't interested in doing that. Do you have some particular reason beyond wishful thinking to believe that will change?
Perhaps you can persuade S/MIME client implementations that your certificates are universally trustworthy, and then you've got the makings of a PKI for S/MIME. But I would not hold my breath.
You're the one who wants this, so it's you that will need to set up a Certificate Authority and then get it trusted everywhere for S/MIME. I genuinely wish you good luck with that.
If you want to build a CA that issues S/MIME certs you can already do that. If you want to leverage ACME you can even do that (using https://datatracker.ietf.org/doc/draft-ietf-acme-email-smime... for example).
Perhaps you can persuade S/MIME client implementations that your certificates are universally trustworthy, and then you've got the makings of a PKI for S/MIME. But I would not hold my breath.