Did you rebuild both the mail and app containers? I think the easy solution is to remove the stuff that tries to give the mail receiver a certificate. Does the cert work for the site?
If the Discourse site is not at inbound.thesite.org that’s the problem. The idea is that the hostname of both the Discourse site and the mail receiver will be the same. You’ll need to find Setting up Let’s Encrypt with Multiple Domains to fix that.
This is rather beyond the intended scope of the mail-receiver howto. If this isn’t enough to get you on track and you have a budget, feel free to contact me.