I somehow lost the ability to post to any part of this forum except for Greenhorn Introductions despite being here since 2014. Perhaps, an admin will want to move this to Planning Concepts.

For self-employed investors who use Solo-401k for retirement investments, using third-party providers allows for more flexible terms than you would typically get from a prototype plan at a major brokerage or through a corporate 401k provider. You can get things such as in-service conversions, after-tax employee contributions, self-directed investments, and debt-financed investments.

When the 401k plan reaches a $250k balance at the end of the year, the investor then has to file a form-5500 to the IRS.

Apparently, this $250k limit is not based on the equity value, but based on the total assets in the plan. Therefore, it's easy for an investor to hit the limit well before they hit $250k equity value if they do investing on margin or buy real estate with their 401k. I'm now filing a late form 5500 for the year in question which unfortunately carries a $500 late fee per form if accepted through the Delinquent Filer Penalty Relief Program (Form 14704), up to $150k late fee per form otherwise. Hopefully, this post helps someone if they come across a similar situation.