Learn about reverse proxies such as Nginx or Caddy. The various selfhosted communities on Lemmy should be able to help with your questions. That’ll fix the port conflict problem. (Though I recommend wiping your Lemmy DB and starting from scratch when changing domains. Federation is really finicky in situations like that)
My second recommendation would be to use blocklists to deny the crap parts as opposed to allowlisting as that will impact your view of the threadiverse. (Which by itself isn’t a bad thing, but is the wrong tool for the job in most cases I’ve seen people asking about it)