I've never heard of Nextclouf AIO, do you have links?
For the RAM, the 200mb free and 7GB available, it's because linux uses ram for caching things, but this ram used for cache can be freed right away when needed (this is the 7gb available)
If i'm not mistaken docker will take all the ram it needs.
In my personnal setup I use Portainer to manage things with a UI and Ive setup nextcloud with the Stacks functionnality of Portainer (it's just multiple docker compose) and in the stack I have Nextcloud server and a Nextcloud mariadb database (and maybe something else ? I might be forgetting some things) have you tried setting up Newtcloud this way ? I suppose the AIO is bundled with the db and the server as one image?