Recently returned but having trouble getting into post 99 content, please advise ...

[Zeargi]

The more things change, the more they stay the same. You can do Ambuscade alone if no one lets you join, but I feel there are a few other things that can be done for gear. Sinister Reign, Skirmish, and Delve have decent gear, but aren't really "On par" with stuff they released recently. But I'm sure if you can get a good MB set for your RDM you should be about to join most things that require magical DMG. Starting next week is an EXP Campaign so you should be able to level another job quickly if you so choose to. (WHM, BLM, SCH, PLD, RUN, GEO seem to be the flavors of the game currently) With all the extra trusts they have for Login and coming up next week, you'll be in a good spot with those. There's also a DRG in my LS that goes to events and does extremely well damage wise, of course he is a Master DRG. I've been soloing stuff on my SMN, with a handful of things I join in my LS (Though I have a tread lately to screw those up).

But most of the things require gear, but when I went through, I followed the flow pattern as it came out:

1.) Sparks Gear
2.) Reforged AF
3.) Skirmish
4.) Reforged Relic
5.) Delve/Alluvion Skirmish
6.) Unity
7.) Reforged Emp 109
8.) Sinister Reign
9.) Vagary
10.) Reforges Emp 119
11.) Escha NMs
12.) Ambuscade
13.) Omen

Some of these will require other players to get things done, but you can get good stuff that can be extremely useful. Omen should be your goal, even though it's not really my favorite right now. But reforging Relic, and Emp should be another goal, because I feel they are also going to get reforged to +2 and +3 like the AF.

Others might offer better advice, but I just do the stepping stones for the most part.

As for 14, I started it and then left and found myself struggling to catch up again when I came back. Very similar to FFXI, but their Cross Server party finder does come in handy when you're looking for help and can't find it.