@Mikhail Petrenko
First - congratulations on getting a great deal in contract! $455k is a great price for 3/2 in San Lorenzo (even in the current environment). I really like your strategy and execution plan. Your rent numbers are fairly realistic. The biggest issue (not a big risk) is that you may not be able to subdivide the lot into two SFRs. It may mean that you can create 2 units on the same lot (and in that case you may not be able to add 2nd ADU). Even if you don't get to 4 units, you can get to 3 rental units and the strategy will still work very well. You may end up spending some more money and time on new construction than the numbers in your budget. And you may not be able to cash out during re-fi as quickly as you project. So keep fat margins in all of your estimates.
I also plan to add ADU to couple of properties that I own in the Hayward/San Leandro. I hope I can learn from your ADU experience! Good luck!