Dillon,
Just looking at the way you wrote your post, it seems like you are leaning towards Out of state investing. The con you listed is probably something you would deal with if you were to invest in state and at some point decide that you want to grow your portfolio. It's not a strong enough reason to NOT do OOS investing.
I have a friend that started Real Estate by house hacking, he was telling me to do the same but I wasn't ready to be locked down to a place or have roommates.
He bought a large duplex, rented out one unit, lived in the other and rented out each room in that unit. From there he slowly did renovations, increased rents every time he got a new tenant, and then started buying other places out of state and in other cities in CA.
In lieu of getting a duplex, get a 3-4 bedroom house and rent the rooms individually OR get something with a floor plan that allows you to convert it into separate units - think something with multiple (front and back) entries.
You could also look at other cities in California in lieu of going out of state. For example Taft is a pretty cheap city, there are many others. just accept that you will be in the middle of nowhere. So you could rent an apartment for now and then buy a house in a cheap city like Taft. That cashflow will put you in a better position to qualify for SD.
I'm looking into out of state investing as well. I'm finding that I can get a nice 3-2 for 140k or less that will cashflow well. I think when first starting out priority should be placed on cashflow. Ideally, you should try to get both cashflow and appreciation, but if a place will not cashflow don't do it.
I'm debating renting my place and moving into an apartment, just to get the mechanics (of managing a rental property) down. But I am seeing that I could also easily just get a cheap place out of state which will FORCE me to learn how to put processes in place.
When I watch a lot of Youtube videos I notice that most of the rehabbers ultimately decide to outsource and put processes in place to allow them to "scale" their portfolio. Otherwise you can burn out.
To answer your other questions
1. Anyway around the self sufficiency rule? (seller carry?) - you can look into Subject to or Seller financing, or get some private or personal loans to make up the difference.
2. Is something like BRRR strategy possible in San Diego? - I'm sure it is, you'll have to do some digging for off market deals. Keep in mind most hard money lenders will want something with at least 30% equity ARV and they may want you to put some money in. If you have good credit you can get a personal loan for that amount.
3. Is it worth looking at SFH's and maybe adding an ADU? - That is an option but I have seen ADU additions run as high as 120k on the forums. So run the numbers to see if that works for you, or shop around. Just make sure it is properly permitted.
That's all I have for now, good luck!