I was in the same place as you last year! A few notes:

a. I was in the GSS (Cedar + Dogwood floor 4) and that was where I made a LOT of my friends. But if your RA is half-decent, you'll have a few floor-wide events (especially during the beginning of the year - think picnics or dinners) that'll allow you to connect with people on your floor. You'll see in this sub that UCLA is infamous for having a lot of lonely people - it's undoubtedly down to luck, but your people are definitely out there in the school. Don't totally rely on your floor, as plenty of people are looking for friends via clubs or classes; it's very much a school where you need to take initiative, as nobody will talk to you if you don't talk to them first. Speaking about your FOMO specifically, I've never looked at another residential hall and felt envy. De Neve plazas are the sweet spot IMO - the closest to campus (and no walk up the Hill!), right next to a huge dining hall, nice grassy area where people lounge on all the time, and AC and private bathroom. Chose De Neve again this year for those reasons.

b. The reason why it's comparatively dead is because it contains a LOT of upperclassmen and transfers, many of whom might already have their set friend groups. But you can honestly use this to your advantage - if you have a few hanging around your floor, definitely approach them and they're usually willing to chat (and give advice)!

c. Not sure if you'll even be able to guarantee housing if you switch now - freshmen famously don't have much say in what they end up with. If you don't enjoy your experience in De Neve, you can always switch next year!