Sub-servers are about ownership and access. Nothing else.
If it’s your system, give them each their own domain (virtual server).
But, as mentioned, a sub-server is just about ownership. It wouldn’t keep you from proxying to a go app server in the sub-server and using php-fpm in the parent server (or vice versa). Sub-servers are just about who owns and manages them. They have all the features of the parent…they’re just owned by the parent account and live in the parent accounts home directory.
I put every domain (and subdomain) on my servers into their own virtual server, because I manage them myself, and I have root access. Sub-servers are about allowing a less trusted (non-root) user to “own” and manage many domains within one account. If that isn’t you, it’s irrelevant.