I could have fully rehabbed that home and sold for $175K but instead I sold as a fixer upper (only needed cosmetics).

he could have or should have made one call to the building department to verify.. 

I also have $100,000 in the bank, and I'm trying to figure out what steps to take next.

Quote from @Phil Johnson: I currently have a duplex and a single family which are doing well.  

I always remember that rockstars know rockstars, so find someone who is a mover and shaker - likely a meetup host - and then ask them who they know.The people doing 100 flips are super busy, so I've also found the best person to find is someone who is a step or two ahead of me...so maybe for you, that is someone doing 2-3 flips a year. 

You have a limit if it is an ADU.

Aside from what everyone else wrote, note that adding units require various steps that you may not have considered (e.g., adding a new street address with USPS, etc.).

Even a small first step will teach you so much, and with your analytical mindset, I have no doubt you’ll make smart decisions along the way.

I have been asking around to different lenders, both Conventional and DSCR, and have been asking what kind of criteria I need to meet in order to apply for pre-approval confidently.

I am breaking into real estate investing and have extensive experience in condo / townhome law and association laws.