[Shadbase]

I, from my own experience, can also say that a portion of it is right place at the right time. You can accidently find yourself in a plotline you had no idea was going on just because you've stumbled across it.
A good way to have that happen -is- to, indeed, just make friends. But it helps to also make OOC connections and friendships greatly. My most meaningful RP only happened because I tagged along on a friend's character idea on a whim and made another character to be friends with their's. That's only an example and doesn't always happen, but... it's certainly proof that the OOC part of the game really does matter. Just meme. Put yourself out there in a positive way. Join the different communities (Hardcore, Milita, SL2 General, etc). That's all I did.

Saw's method I find ALSO works EXTREEEEEMELY well. You don't have to... vomit centipedes, but if you can figure out an interesting hook?
Walk into a town and get people into it, or put up an LFG. That's how I got 90% of my RP when I first started my (current) main character. I just threw up an LFG and people I'd never seen before just showed up and stayed for the RP. Now a ton of people just... recognize me - or they've heard my character's name. I can mention their name in LOOC on other characters and have people immediately recognize it.
I also find that if you can find a less-than-generic playby that isn't a typical anime woman/girl or man/boy, people RP with you more often. That's just an observation, based from the fact that I usually wait to make characters until I have a visually interesting playby now-a-days and they usually get good enough interactions and friendships.

I flirt, I offer drinks, I ask for spars, I get into conversations, even play drinking games... And I get a lot of surface-level roleplays with random people.

The only problem I think you could be having is that... this is exactly how you get surface-level roleplay with random people. Those people - more than likely - think you just want casual RP and so aren't going to bother going further.
My suggestion, genuinely, would be to pick people who have characters you're interested in yourself, and try to further your relationship with them to where they would be more interested in deeper, more meaningful RP - but keep yourself open to further connections. My main has their main friendgroup, family, and partner... but I keep making friends anyways!
You have to remember that the effort you give is usually matched back - not to imply that you're not giving any effort because I know EXACTLY how you feel and I used to do the same exact things to try and get people interested in my character - and if you do casual RP, a lot of people are only going to give you casual RP.

... I might be rambling, but there's some genuine stuff in there, I promise.
And... I like Karvan, to be honest. You flirted with a 70 year old woman - mad fucking props lmfao. <3