If you don’t have plans on what you want to do with them, just leave them alone to rot in your barracks. Vanguard is pretty good fodder, but don’t unlock potential unless you have a plan on what you want to do with them or their fodder… If you want to inherit Firesweep Lance+, it’s better to wait for a Cordelia.
Thea doesn’t have much going for her: she’s one of the worse ftp lance fliers. Doesn’t even have great fodder potential.
Cynthia is a worse Shigure, but she’s also a unit that costs grails to get more of. For that reason, I’d say don’t fodder Cynthia.
I have a Cordilia but she has -spd
Do you specifically want a lance flier for use? Florina is an enemy phase lance flier, Est is a powerhouse for flier emblem, Shanna has that awesome Slaying + Wo Dao effect… I wouldn’t promote Thea or Cynthia. There are plenty of great F2P options.