I don't have any experience with Optima, but they advertise themselves as a tax relief & resolution company. Some quick research indicates that they are reputable.

Optima Tax Relief Review 2022: Pros, Cons and How It Compares

If your mom already signed something with Optima, then your father is likely committed to their fees. He should, of course, understand the breakdown of what he's paying. For example, is that $20k all taxes? What portion are back taxes and what portions are penalties & interest? Also, what are Optima's fees?

There's nothing wrong with using a service like Optima. Your dad was, essentially, operating his own business for 2018 and 2019. If he paid $0 taxes, then $20k owed over two years is not an unreasonable amount of money to owe. That's around $10k each year.

Working directly with the IRS is fine, but as I mentioned, if your dad has already signed paperwork with Optima, at least some of the fees have already been committed to. What you need to understand is where they are in the process, and what fees are yet to come.

Navigating back-tax issues can be confusing and difficult. While the folks here might be all to willing to go directly to the IRS, you should understand that it's your dad's responsibility to get his filing in order. The IRS won't do that for you.

Taxes are unavoidable. They cannot be discharged in a bankruptcy. Your dad will ultimately end up on a payment plan. If he doesn't make the payments, they'll garnish his wages. He's got to figure out a way to make ends meet. He doesn't have a lot of options at this point.