I have been a w-2 employee until recently, investing in real estate on the side. I've left that work, and plan to self-manage apartment buildings I own, formalizing that as a 1099 employee of each LLC. All LLCs have partners.
What are my best options to contribute to retirement and maximize tax advantage, either near or long term. Age 42
I agree that the Solo 401k is likely the best retirement plan for you if you are eligible. It allows for investment into real estate and other alternative assets. It also has quite a few benefits over a similar retirement structure, the self-directed IRA.
Compared to an IRA, Solo 401k contribution limits are about ten times higher. Since there is no custodial requirement for the 401k, you don't need the additional expense and administration of an LLC to obtain checkbook control. You can take participant loans from a Solo 401k, but the IRS does not allow this from an IRA. Built-in Roth components, spousal contributions, and an additional tax exemption also apply to the Solo 401k. Solo 401k plans are often quicker to set up and cost less money over time compared to most self-directed IRAs.
I would recommend reaching out to a few plan providers to learn more about the structure and to see if you are eligible.