It is possible to borrow from an employer plan like a 401(k).Some people claim to "borrow" from an IRA by executing a 60-day rollover.  

I want to roll over my former employer's 401K into a solo 401K so I can more flexibility wrt investments.

Yes, you can do both at the same time, ie 1) contract to purchase in the form of an option to buy within 24 months, with a 1-year rollover clause, and 2) lease for your preferred term, say 24 months. 

Or to justify the creation of a solo 401k where you can rollover other retirement plans that you have.These situations need to be modeled carefully to determine their cost benefit ratio.

But, I do miss my students, sometimes I think of them when I roll over or book trips. 

Check out something called a Roll Over Business Startup (ROBS) account.

Flipping of course can alleviate some of those concerns, but long-term wealth creation for most people will be done through long-term property ownership (some of our students at FlipSystem who have had success flipping are now using those profits to roll over into long-term rentals as well so they can benefit from some of the tax benefits and appreciation of property ownership).A collection of 3 properties, especially 2 in the Bay Area (I’m assuming those are definitely not cheap), is an amazing start to a portfolio!

Maybe there is some overlap or specific skill set you have that would roll over to real estate investing nicely!

Rollover from your old employer 401k into self-directed Solo 401k is not a taxable event and wont' affect your taxes.

Started investing in real estate during Q4 of 2021 because I needed a place to roll over a past retirement account.