gogetssl.com issues free 90 day Sectigo (formerly Comodo) certificates and they have an ordering API. Caveats: 1) I don't know if those certificates will work in old VLC clients or whatever. 2) After you order the certificate you get an email from the CA with a link that you have to click saying that you approve issuance. I don't know what happens if you try to automate that.

For me the main hassle of LetsEncrypt is the 90 day rotation and there have been situations where I'd rather just pay for a longer lasting certificate. Gogetssl (above) sells 5 year DV Sectigo certificates for $16, it looks like.

Ignore the prices shown on the not-logged-in part of the site: sign up for their "reseller" program (you get approved right away automatically) and you can see their real price list while you are logged in.