Dave, have you checked to see if the domain you used to have is available with another TLD? Just a thought.
As a side note regarding using subdomains for categories, each time you create a new category you have to create a new subdomain… and either create a new CA for that subdomain or amend your current CA to include that new subdomain. If you go with amending the CA and mess up, all the subdomains (your entire forum) will lose https until you fix it. Sorry, but I think using subdomains per category is just asking for a very big headache.