Hello Alexander:
There are four GA type airports in and around San Diego:
KMYF: I know of two RV-10's that are based there. I am sure there are other experimentals as well but I do not spend much time there. As a builder, typically you cannot start there due to the surrounding neighborhoods and need to do phase 1 elsewhere.
KSEE: There are a few more experimentals there than KMYF. Several of them are on this forum. Similar issues I believe building here as phase 1 needs to be elsewhere.
KSDM: Brown field is down by the border and the home of one of the two local EAA chapters. You can build and start Phase 1 from here. Do not personally know any builders there.
KRMN: Ramona is about 45 minutes east. I am building my RV-10 here. There are many other RV's at the field with a very active experimental community. EAA chapter there along with a place to do Phase 1. There are at least 4 or 5 builder/owners on the forum.
Stop by some time or drop me a line and we can chat
E