Should I create a sub-account for myself or for each of my own brands or niches?

Usually not. Your agency account (what you see when you log in) is the account you work out of. It is also the master account with special powers: it can create sub-accounts and push down snapshots (templates).

Use sub-accounts for clients, so each client's contacts and data stay separate from yours. A demo account is also a good use of a sub-account, so dummy contacts do not matter.

A sub-account for yourself makes sense only for a completely separate business you will keep apart, for example a car wash alongside a marketing agency, or a brand you plan to sell.

If your businesses or niches are related and you may cross-sell between them, keep them in your main account. Separate sub-accounts mean working out of several accounts and inboxes and switching back and forth, a marketing blast to your whole list means working across accounts, and you can run into issues using the same phone number, domain or registration in several accounts. Instead, separate niches inside the main account with a different pipeline for each niche, plus tags, saved views and custom fields. You can have as many websites and phone numbers as you want in one account, so each niche can have its own website, domain and numbers, and you can work the whole inbox at once. Multiple A2P (10DLC) applications under one business are not a problem.