We have a number of people in the linkshell that have no jobs 75+ yet (they made new characters), so there is a fair amount of partying that goes on in the 40/50/60 range. (A lot of the time it's just duoing/trioing, but they get good exp and with more people they could get an actual party going on.) As for population, I'm sure you're already aware that Asura is pretty much the most populous. I can't speak to the number of people pre-abyssea who LFP, as I haven't done that in a while, but your chances are certainly better here. Plus, there's the LS people.
As for getting into Abyssea at 69, it really depends on the leader. You'll definitely only be able to get in to the Visions areas to start with, but that in itself isn't an issue as long as you have atma to make up the difference. Your best bet will be to look for a shout in PJ and tell him you're 69 but have atma or something. Some people will take them as low as 65, and you'll be more likely to get one as DNC since you can be a backup healer if they are low on whm (a common problem these days).
Moving servers is pretty intimidating at first, but from my personal experience doing it, it's not that bad. I have never once regretted it. I have a lot more really good friends on Asura than I ever had on Lakshmi. It can be daunting at first, but you'll be at an advantage because you can jump in with the linkshell, whereas I started completely from scratch and knew nobody.