What forms do I need to fill out for the IRS for my Solo 401K?

No annual tax reporting is applicable until your assets reach $250,000. At that point, you must file a Form 5500-EZ.

However, you must always file a Form 1099-R with the IRS should you take a distribution of your plan assets upon plan termination or during participating in the plan. Form 1099-R also applies to in-plan Roth solo 401k conversion of both after-tax and pretax funds.