I’d really like to supplement my efforts, by finding wholesalers that are active in Texas and in other states.

Supplement the historical performance with the relevant current and forecast statistics to derive the odds of continued long term appreciation.  

Now add in all of these above factors and consider how many times you will have to submit conditions to the lender for each month of new pay stubs, bank statements, verification of employment, credit supplements, builders invoices, appraisal updates, rate lock extensions etc...Your looking at 4-6 months now in most cases.I would suggest that after 2021 settles in a bit and "Hopefully" Covid starts to clear it's name form the media.....You might look back into that option.  203K is a time investment loan option that I think could be a last alternative choice to something quicker like a fix & flip loan. 

My goal is to create passive income to supplement my retirement in 11-15 years.

You have a steady flow of individuals going TAD to NAS Pensacola and Corry Station for training (safety school, helo dunker, etc), but you also have a great market for vacation rentals in the spring and summer to supplement the military individuals who need STR's. 

@Brandon Jefferies,@Dalyn Hazell's answer is extremely helpful.Coming from the lender side, I'll supplement Dalyn's first answer to your question,How to find hard money (or other types of) financing?

I certainly think we had the potential to maybe do another (5) but beyond that I am at a loss to understand how people are doing it.Some background is that most of these houses are what I call light rehab (i.e. cosmetics, flooring, maybe a roof) and we do a mixture of self-labor along with finding supplemental workers where we can as professional contractors seem to be impractical somewhat from the economic perspective but more so lead time. 

I am considering building an attached ADU to supplement my income.

Here's our interesting situation:  we have grown to roughly 13-15 rental units depending on how you count, but we're also starting to integrate more flip projects with the main intention of supplementing our income picture, and ensuring access to capital. 

However, it will be supplementing your living costs.