@Christina Greaves already some very great input from the group here. I'll comment to each of your points below:
1) Personally, I focus on the boring, conservative stuff...long term, buy and hold multifamily in landlord-friendly states. All of my LTRs are in the greater Boise area but I watch CO, OK, KS and others. I own a 3-unit STR/MTR in North Tacoma but I don't plan to grow that part of my portfolio. Maybe one STR condo on Oahu where my brother lives and has a real estate office. I believe it is ok to diversify but the more you focus, the more successful you'll be. Thankfully we all don't focus on the same segment of investing! :)
2) I don't focus on zip codes. I watch for properties in specific metro areas, then once I identify a property I'll do a deep dive into the neighborhood (even that particular block) to verify crime, development, etc.
3) AVOID SEATTLE and possibly now Federal Way and Burien for LTRs. They are continuously adding regulations on top of the existing state landlord-tenant laws. Be sure to watch other cities' discussions about their local ordinances being proposed by such groups as Tacoma 4 All. That being said, I know investors still bullish on Seattle, Tacoma, etc. It's all about mitigating your risk. RHA is great for keeping its members updated on long-term rental ordinances but if you intend to invest in STRs you'll need to specifically watch local city council announcements about potential regulations.
4) Resources: A knowledgeable, experienced agent. Local investor/landlord groups such as RHA, REIA, Facebook groups. Websites: Spotcrime, Rentometer, Airdna...are a few examples. Also look at the ipropertymanagement site and click the "research" button at the top right. I don't overthink the analysis. If the numbers work and I like the area/property enough that I would live there myself, then it's usually good enough for me.
Best wishes on your investing journey!