There's a lot of other things to consider here but I'll go through your list and make some suggestions based on 10 years of investing across various asset classes - single family, multifamily, industrial, storage, retail, office, etc. and I've had experience with section 8 housing, BRRRR, flips, etc.
- Purchasing rental properties? (Best regions/areas for returns?) -- sort of depends where you live and if you want to invest out of state / have the ability to manage a team remotely, the best out of state investors are very active in managing their teams. I like the midwest (I'm from Milwaukee) but there are great linear markets here with solid cashflow and less volatility.
- Flipping homes? -- if you're looking for a way to get your hands onto more capital quickly you could certainly do this. I'd start small and just try to learn the game of flipping.
- Acquiring pre-existing rental portfolios? -- solid strategy, make sure you look at the trailing 12 months cashflow and do extensive due diligence, you might even consider hiring someone to help you with due diligence
- Investing in mobile home parks? -- solid strategy as well, if you go this route I would consider investing with an operator that specializes in this asset class. My friend Dylan Marma would be a solid bet and I'd be happy to make an intro.
- Using leverage to scale, such as borrowing against this cash for larger down payments? -- depends on your risk appetite but leverage can certainly pour gasoline on your real estate investment fire in both a good or a bad way.
- Employing the BRRRR ? - tried and true strategy, start small and learn
- Targeting Section 8 housing? - just starting out I would steer clear, it's nice that the rental income is guaranteed but that can be a rough tenant base and many new real estate investors just get smoked
If I was just starting out in real estate and had $750k to play with, I would really try to understand my goals - is it to retire on a beach and be completely passive? If so, I'd 1) join a real estate investing mastermind that focuses on your asset class of choice (multifamily, industrial, storage, etc.) and learn as much as I could - it's worth the $10k-$20k in both knowledge and connections. Then I'd probably spread that $750k out amongst numerous operators and diversify across asset classes that I like (for me it would be industrial, multifamily, storage, and mobile home parks). I would target a 2.5x equity multiple and 8% cash on cash return, but I would focus mainly on the operators and try to figure out if I believe in the team and vision, and really try to understand if I trust them or not. From here you could decide if you want to stay on the passive side of things as an LP, or if you want to begin raising capital for your own deals / funds based on your track record of investing with other experienced operators and being part of the masterminds that you're in. Theoretically you could turn that $750k into $1.5m, and then turn that $1.5m into $3m within ~15 or so years and be earning 8% on your equity along the way, relatively passively, and it would allow you to build a track record to start doing your own deals.
Just my two cents! I have resources available for you if you're interested just reach out via DM.