Now I'm really confused; I just tried again with a new domain and this time strictly followed the Beta invite's email instructions (with letsencrypt-auto):
/root/.local/share/letsencrypt/bin/letsencrypt certonly -a webroot --webroot-path /usr/lib/mailman -d lists.garbage-juice.com --server https://acme-v01.api.letsencrypt.org/directory --agree-dev-preview
and while it did warn me of the deprecation of the agree-dev-preview command, it worked, for the new cert.
Now I just tried renewing the same cert i just received, this time adding --renew-by-default, and it also now worked fine, while still warning me of the deprecated command.
So now I went back to one of my original domains (one cert, 2 domains) I had been trying to renew for days now (as a test), and SOB, it appears to possibly have worked except i got rateLimited (probably due to my testing).
So IDK what's up but it works now seemingly. WTF!? haha........