most of your investments are in tax deferred/tax advantaged accounts (401k, IRA, etc)
mortgage, car loan, etc
married filing jointly, head of household, or single
modest amount of charitable donations
And here are some things I consider “complex”:
married filing separately because you think it’s more efficient for you
owner of a business
multiple real estate investment properties
lots of after tax investments (5 figure profits on an average year)
1099 income with lots of expenses
regular gambling income or other “irregular” income
The vast majority of people have “simple” taxes, so you really need a good reason to think you need a professional. That said, paying once and having them walk you through it can be educational.