Legal Question in Business Law in California

Re: what domain should I legally register as DBA?

You should incorporate to provide liability protection over your personal assets. The corporation can be structured to control the domains in any way you like, with DBA's as necessary. Contact me if interested in doing so.

Re: what domain should I legally register as DBA?

The test here is whether you are doing business under the name. Not every domain name is necessarily going to be "doing business" in the sense that the public will be buying from, or selling to, a person or entity "fronting" behind that name. However, you should register each and every fictitious name under which you hold yourself out to do business with the public.

I think (but I'm not sure) you can save time and expense by publishing all the names in a single advertisment.

As to insurance, you need to speak with your agent, but more likely than not each operation can be covered under a single policy with a single premium reflecting the combined exposure. Be sure you are personally covered as well. The size of the single premium will reflect the combined risk from all the covered DBAs.