But we also know that the materials he employed on the Usonian houses are not exactly affordable by today's standards... 

California's housing prices are too high for our budget, and I'm at a loss.I'm seeking your advice on alternative ways to use these funds for good.

Slowly build out a repeatable process...if you continue to change things (as in different types of properties, different levels of renovation, different finish material, different neighborhoods, etc.) you create more variables and make it harder to solve to the problem.I am a math guy and less unknowns/variables you have the easier the problem will be to solve. 

I would look for a custodian who does not charge a fee based on your assets.I normally see the costs being A one time set-up fee along with an annual reoccuring fee.The thing you want when opening up an SDIRA is that the return that you will make with alternative investments will be greater than the fee that is charged to you for the SDIRA plus the return you would have likely gotten with a regular IRA.

Issues we faced were conflicting opinions on the next home we should purchase for our portfolio and what upgrades to do when we were doing a flip, the cost of labor and materials, etc.

I plan to check them out although I don't think they can be competitive for lending against equity vs alternatives like IBKR.I will keep this thread posted as we navigate this in case others are interested.

Alternatively, you could explore house hacking with a multifamily duplex, where you live in one unit and rent out the others.

Alternatively the debtor can payoff the note either in full or at a negotiated discount.  

I stopped at the "right" part.You need to start interviewing PMs in Austin, getting a feel for prices to renovate(hvac, water heater, floors for material + labor, etc.).

An annuity is definitely a different level of risk exposure than this type of an alternative investment.