Elson is great for the def/atk(when awakened) buff. Tieria is also a great 3* that boosts readiness (atk bar), def breaks on s3 and blinds on basic attack. Other notable low star units to keep are kris (3* archer that stacks poison, very useful for bosses but crap for campaign) and carmainrose (very solid mage dps/tank).
The game lets you reroll 10 pulls 30 times (of which you keep one 10pull) after clearing chapter 1-10, and you can reset your account to reroll again without having to download all data again. If you want to reroll hardcore, you get 8-9 pulls after clearing chapter 1-4, which can get you a 5* unit on top of your 30 rerolls, including 3 newer units that are not present in the 30 rerolls (like Krau, Haste (and Yufine I think

. The rate for a 5* unit is 1.25%, so it takes a bit of time but with something like Nox you should be set in a couple of hours. Imo if you're going to be f2p, spending a couple hours rerolling for a second 5* is worth it.
Destina and Sez are regarded as the units to pull for, but don't be afraid to keep other pulls that have great synergy, like a 4* healer/cleanser (Achates/Angelica, even Lots) and a nice dps (Ravi, Charlotte (which imo is more versatile than Sez and more future proof

or 2 dps that work well together (I kept a Chloe from 1-4 together with a Charlotte from the rerolls f.ex.).