You can pay for financial planning expenses with your Solo 401k funds but you shouldn’t.
Your retirement funds are special in that they are tax-deferred. That means the have special tax benefits and when invested can grow much faster than regular funds where taxes have already been taken out.
If you use retirement funds to pay for financial planning expenses, you are reducing the amount of tax-deferred money to invest. That means you’re taking money away from funds that have tax benefit protection, which can reduce your compound growth and profits for your financial future.
So, while you technically can pay for financial planning expenses with 401k funds, it’s better for your long-term profit to use non-retirement funds.