Hi Raphael,
Investing in Austin has a lot to do with its appreciation. It is hard for many on the site to invest that way because they have not been in markets like Austin that has had 40 years of pretty much straight appreciation. The economy here is setup differently then most places. We have a huge government job base being the capital of one of the largest states in the nation. We also have a huge university job base here. So when times get tough, the Austin market does not dip like others. Even in the housing crisis our prices fell just 1.9%. So Austin is more about appreciation and ROI then maximizing cash flow. If you can get it you will make way more money than those maximizing cash flow. But to your questions...
-SFH vs duplex? Really depends on what you are trying to do. Want to house hack but not really wanting people in the same house, then duplex. Want the most appreciation, probably SFH but the multi-family properties available in Austin are down like 70% from about 2 years ago. So that kind of scarcity has boosted their prices too. More cash flow but more problems as well with duplexes. I own both here and happy with both.
-ADU potential for a SFH (7500 sq ft plot)? I have yet to really get into this simply because the building costs have skyrocketed making it just not worth the expense. Maybe when that comes back down that will change so it is nice to have a lot that is big enough to have one.
-Buildable land in general- again, building costs have skyrocketed so it really is hard to find land cheap enough to where it because beneficial to buy and build from an investment point of view. Of course if we have another year like last year which I believe we will, then building could be ideal.
-Other zoning issues? Depends wat you are looking to do. STR's are very hard to do in Austin. Zoning will change here soon as it has been on the table for a few years now and they are trying to open areas up to allow more multi-family.
-Newer build vs “needs work”?-Neighborhood/schools? You pretty much make twice what you spend in equity. That being said, you may not want to deal with it all. There is a value to having not have to do anything.
-Proximity to rail? Important but not critical. Texans are not really rail people but it will be more and more appealing as the population continues to grow.
TLDR: I realize that long-term appreciation will be a huge factor in my housing finances (once I eventually sell this thing). What should I be looking for to maximize this? Jobs, jobs, and jobs. Follow the job growth. Tesla, Apple, Facebook, UT Medical Research Center, Oracle, Amazon, Google, Samsung, Micron, and 1000 more. Where are they going and how many are they hiring and at what salary. Other things are like commute times, projects like Leander's Lagoon project, shops and restaurant areas like the Domain or Apples future campus.
Hope that helps!