However, if this is during an economic downturn when default would likely happen, there might be no equity in the house for the original seller, and I can't see how a seller would be comfortable being ultimately responsible for a mortgage that might last 20-30 years.

Of course, it ultimately depends on your available funds, short/long-term goals, etc. 

I still think this is what's bound to happen ultimately.

My ultimate goal is of course make a bigger profit.

Ultimately you need to put in what the masses will like and not what you like.I do a lot of contemporary designed homes and some of them get white shaker cabinets while others get grey shaker or some other contemporary style and color.

Ultimately, you want to make sure the math works and cash flow is positive. 

So, I have no idea how much money I owe for 2023&2024, how much money is actually mine, how to take advantage of tax deductions/credits/RE professional Status/ renovation costs/perform a cost segregation, if I really need a wealth manager at this point and how to take out my retirement investment and move it to a mutual fund I run independently.My ultimate goal: create systems for my corps to run smoothly and optimize business-related expenses,  keep my books up-to-date for both corps so I know my input/output, pay quarterly taxes throughout the year, give myself a modest monthly income and  continue to preserve/build wealth and grow my RE PORTFOLIO!

My ultimate goal is cash flow to be able to quit my job (self-employed) and focus on real estate full-time.

I have found time is better spent building tools to quickly analyze & procure new deals, rather than hyper-analyzing the metrics of one deal, ultimately making initial review faster & more reliable (spend time on the things that make sense, quickly remove things that don't, even junior-level staff can identify good opportunities with the tools, etc).For example, if I'm modeling a SFR to 3-4 unit scheme, I can quickly check the $/sf/mo for each of my proposed units and get a good idea of the GOI without having to manually dig through &  find rent comps in that City/Zip:- So instantly I know in Woodland Hills that a Studio is close to $5/sf/mo on avg, 1 Bedroom closer to $4/sf/mo, with the 2 & 3 Bedrooms closer to $3/sf/mo.

Ultimately, my goal is to wait for rates to move down and lenders to allow HELCOs on investment properties with a fixed rate.