What servants should I focus on upgrading?

Iri and Martha are useful for their guts and debuff cleanse respectively. Cu is nice if you have the spare xp and can clutch some fights.

Hello, fellow F2P, nice Scat and Sabershiki you have there...

Level:
-Mash (and her first 2 skills), her 0 cost and defensive buffs are actually very valuable, specially against zercs (she is neutral to their damage), in Camelot her NP and first skill will have a monstruous buff.
-Euryale (best anti-male unit, charm in the NP, powerful charm in the skill, overcharge effect of NP is extra damage against male), against male sabers and berserkers, you can never go wrong with her...just level the skills (specially Whim of the Goddess) the maximum you can. And do her interlude to cause more dmg on NP
-David, his second skill is amazing, and will save you a lot of times, his ST Buster NP is solid as hell, for a 3*...

Saber Lily is a bad wellfare, Caesar is better than her, and has lower cost, his skills benefit the team, level those as well, at least to 6 (or 4 if you have material shortage probs).

Saber Gilles is the worst Saber in the game.

Level Protection from Arrows and Battle Continuation of Cu to 6, at least, and third ascend him lv 50, at minimum. He's the perfect last man standing, even having such a low attack and meh HP

" I've also been holding off on upgrading caster because I believe we will get a much more powerful one during the Halloween event (Elizabeth Bathory?)"

Yeah, but she's AOE NP type. Level Medea (and put her first skill at 4 for instantaneous NP 100% gauge), as she's the best ST Caster who is not gold (and even among gold, there are so few...). Her NP dmg is bad, yes, but CEs can fix that (Heavens Feel, Angels Song, Formalcraft or Black Grail). Also, do her interlude for NP dmg upgrade.

Level Caster Hans as well (if you have a NP5 of him), he the best support, aside from golds, NP and skills are a blessing.

The other low star Casters are truly not that worthy of time.

Also, end full levelling Chloe.