Hi Reth,
looks like you have a good unit base so I guess if you wanted to focus on one of those low lvl plat units I would think either Elizabeth or Anelia would be you best bet. I have heard Elizabeth is a good unit and it is always good to have a vampire hunter at the ready as they do have there uses. Other than that where archers can take places of most pirates, pirates also have there uses and Anelia's skill is a good one to have (I have yet to lvl her as I lvled rachel over her.).
Lilia is a good princess to have but not that best, I see you have olivie so you don't really need to lvl another princess unless you wanted a more tankey princess which is what lilia is good for. Fedora is another good healing unit, her increased range after AW makes her good for certain situations where ranged spots are numbers and you need to cover a large healing base.
My Elizabeth I haven't lvled and don't need to at this stage as I already have Khuri as my Hunter and same with Anelia as I already have Rachel. Not sure if you have replacements for either of them but in my opinion they would be the best units to start with. Though I am not a veteran at this game so others may have better opinions than mine.
Best of luck with you leveling!