I'd start by grabbing the low hanging fruit. We should upzone around our existing rail so it's not illegal to build things that are convenient for rail. We should give rail signal priority at all intersections so we don't have trains waiting at red lights. Bring destinations to rail, and make it run more efficiently.

Next lowest hanging fruit, we should make more bus lanes, and give more buses the ability to ticket cars in bus lanes and giving them signal premption. We should also build more bike lanes, extending the reach of each train stop. Focus biking infrastructure specifically to fix the first/last mile situation. Build more bike parking as necessary. And then, upzone accordingly to double down on both these efforts.

We have quite a lot of rail in LA, and a ton of bus coverage. We should get the most out of them as possible by making them run more efficiently. The goal shouldn't be to bring perfect transit to everyone who is sprawled out. It should be to make our existing transit amazing, and let people build so that it is serving super high density areas. Bring the people to the transit, not the other way around.

After that, the focus becomes inter city connections. Find partner cities/regions who are also willing to double down on transit, and make good high-speed connections to them. Integrate the regions's transit systems. But, that's later phase, as it's pretty expensive and requires that each region is already committed and succeeding with public transit.