Check out other online guides available here: https://www.biggerpockets.com/guides3.

However, the ROI on the cheaper house seems kinda low if I rent out the property as a whole.

I am willing to manage it on my own.If I sell, I can put it in an index fund or towards my current primary residence mortgage with an interest rate of 6.125%.

How are landlords in SF handling the security deposit law that requires them to pay interest to residents on their deposits?

I've always planned on purchasing a bigger multifamily (20+).

The biggest challenge I can see for this is that I hear it's a pain in the *** for the seller to go through a local community bank and confirm their income for their approval for a mortgage, and there are local/community banks that will approve 95-100% LTV loans based on how profitable the deal is.

This is how the rental income is reported on their personal tax return."

Is it better to use that money as a down payment on one property or split it and use it for a down payment on 2 properties?  

My partner rents out her family's vacation home on VRBO. 

If it's because of the tenants, you now have limited visibility on it and difficulty with tenants.