There's lots of opportunities in developing neighborhoods, but also seems a bit saturated at the moment.Also, definitely budget security (remote control cameras, alarm system, etc.) for your properties, as there are a lot of people who occupy themselves by breaking into houses and stealing anything they can sell.Lastly, there are a ton of contractors looking for a quick buck.

My first self-developed software gave me efficiency.  

They plant flowers, keep the yard nice, and I really enjoy them.

I also don’t like renting really nicely rehabbed properties, most renters don’t take care of properties as well as we would hope.

So, your focus may need to pivot from high rental cash flow to little to no cash flow as a rental with an appreciation upside.I would recommend taking your $150K to $200K and securing a solid equity position in something that will cover its expenses and maybe provide a small cash flow, but then is poised to appreciate nicely over the course of the next several years. 

I am considering working with a property developer in NJ to buy land, build a duplex/triplex and sell it (6-9 month time horizon).  

Very nice lady.

Quote from @Marcus Ball: After struggling with local rehab properties in California, I switched to build-to-rent developers and chose a 1031 exchange.

Instead of a traditional down payment, I would give them some money (maybe just to bring the loan back to even).The upside if taken over the loan at the current balance, with some rehab and hopefully getting the front lot back would be a nice ROI. 

Quote from @Daniel Huang: Hi, I am trying to transition to develop my rental income as I am in the final years of my professional career.