;age of the building;have there been any updates to the building envelope over the years (insulation, vapour retarder/barrier, fenestration, etc);are the radiators the older, freestanding cast-iron type, the mid-century baseboards, or the more modern flat(ish) panels;are the hydronic heating lines copper, black iron or {red} brass?

I still have a W2 but our portfolio has been consistently growing, since I started networking more. 

If we sell, our plan is to take our time to look into getting a multi family property maybe just outside the Bay Area (it's too expensive here) so we can continue to let the money grow.

Congratulations to your growing family!

It is known that the house flipping industry is growing and that to be successful in the business of flipping, you will want to buy homes at a discount in order to have nice margins when you go to sell. 

This is the point which you have it all ... freedom, time, money, and a growing business.   

What are some strategies I can use and/or implement to reach retirement strictly off of REI before age 40?

Generally growing population is good, but I also don’t think minor decreasing population is that big a deal.

We are always looking to meet more local investors so we can all get better together and grow in this market.

We would love to connect with other local investors like yourself to grow business, share insights, and simply do more deals.