It seems what I am trying to do is not supported by most DNS services out of the box, and I don’t think it’s fair that I had expected to do it in MIAB. There’s nothing wrong with MIAB the way it is. I learned I am trying to use “cname flattening”, something that seems to mainly be supported by Route 53 and Cloudflare, and if I need it, I should probably just use their DNS externally https://blog.cloudflare.com/introducing-cname-flattening-rfc-compliant-cnames-at-a-domains-root/
For people who require “cname flattening”, it is a possible workaround to use a www. cname, and redirect nginx to the www cname. Chances are though, that running MIAB upgrades will overwrite your nginx settings I am guessing… Have fun with that.