Because you're waiting on your friends to complete the story, you won't be able to do a lot of level 50 content, unfortunately. A huge amount has completion of the Praetorium as a prerequisite to appearing, including almost all of the 4-man dungeons and Hard Mode trials onwards. There won't be a good source of Soldiery or Poetics for you as a result - the first farmable ones are the Main Story roulette.

Otherwise what's been listed above is true. I think you can go Hunting and join hunting parties without completing Praetorium, so that's by far your best bet for tomes and marks.