I need to add more space to my personal Dropbox account, without upgrading to a team.

For individual accounts, storage is capped, even if you’re willing to pay more. Once you hit that limit, the only official way to get additional space is moving to a business plan, which is priced for multiple users, not one person. There’s no option to buy extra terabytes beyond the personal max. Practically, the choices are to clean up files, move some data to another service, or accept the business plan even if you don’t need extra users. Many people with large photo/video libraries end up splitting storage across services for this reason.