I never hand my taxes off to a tax expert.
If I run into a problem where I can't interpret the tax code myself, I'll schedule an hour appointment with a tax expert to get the answer to that specific question. If their answer sounds too good to be true, I'll schedule some appointments with other experts and take the most pessimistic answer. Ultimately, it's my responsibility that my taxes are correct, so I need to understand how they work.
I switched off of using tax software and do my taxes by hand, including rental income and stock sales. I'd estimate my tax return is 20-30 pages at this point.
If I ran my own company, I'd hand my taxes off to a tax expert at the point where filling out all of the forms represented such a significant investment in time that it detracted from my ability to run the company, but they would have to be significantly more complicated than anything the average person encounters. I would also hand them off to a separate firm if they reached the point where I needed credentials for the tax return to be valid - such as to assess the value of inventory.
It really doesn't take that much effort to learn how to do your own taxes, and the VITA program provides good free training in exchange for volunteering, which provides essential practice time anyway.
>where I can't interpret the tax code myself
That still leaves you making the crucial decision of when to roll in the expert with a layman's knowledge base & judgement.
Conceptually "I'll know when I don't know" is a very weak approach, especially on things like tax where the gotchas can be arbitrary & non-intuitive.
It's actually pretty simple. "Where I can't interpret the tax code myself" means "where my attempt to interpret the tax code leaves me with two or more possible interpretations."
The tax forms are very comprehensive. I go through each line of the 1040, following the instructions. Wherever the 1040 references some other form, I find that form, go through every line and follow the relevant instructions there. This approach has even gotten me through obscure edge cases like a company over-contributing to an HSA.
The tax code isn't magic and isn't full of gotchas. The rules may be arbitrary, but they're easy to follow and the IRS won't punish you as long as you're making a reasonable effort and report all of your income. There's an IRS help line you can call. People only really run into trouble when they try to take absurd deductions or claim that whatever tech investment strategy they've invested in doesn't count as income.
If you're scared of filing your own taxes, I really recommend volunteering for a local VITA program for a year.
As a generalist myself, I have decided not to require myself to become an expert in some tax issues, and so as a corollary decided not to complicate my finances with things that could invoke those issues.