Skadi or any of the big 4 support casters would be great for you, seeing as you have none of them, and Skadi especially would synergize well with Kama / Spishtar / Frankenstein. The only problem is that Skadi’s only banner next year comes in November, in JP’s 18M Download Campaign, and by that time, Castoria is less than 7 months away (2022 late June / July). If you also plan to roll for her, your Skadi will get about 7-8 months in the spotlight before being displaced. Not saying she won’t be useful, because Kama and Spishtar, plus whatever other Q servants you get (most likely including Parvati), but her relative usefulness will decrease dramatically.
Arjuna Alter is the other big DPS servant releasing this year, and even if your Buster support is lacking, he’s an incredibly strong SSR Berserker, and even without Merlin, does crazy amounts of damage, while also debuffing enemies. Frankenstein and Tamacat are both kinda fiddly to make effective, and that 30% battery is incredibly useful.
I will also put in a plug for Waver, as he gets a rate-up near the end of April during the Case Files event, and his team utility is, frankly speaking, ludicrous. For newer masters without Kscope or serious charge support, being able to plugsuit in a 50% battery, with offensive, defensive, and crit buffs is absolutely dreamlike. If you’re willing to wait another year to grab him from the SSR ticket in April 2022, feel free to do that, but that’s well over a year of farming without one of the strongest, most general supports in the game, who remains relevant even into Castoria meta.
In terms of welfares, we will get Chloe for ST Archer, BB for Moon Cancer and Arts Stall, Ryoma for ST Rider, Gray for AOE Assassin, Kagetora for ST Lancer, Jalter-Zerker for ST Berserker, Saber Hokusai for ST Saber, CasShuten for ST Caster, and Santa Nightingale for AOE Archer. That should fill most holes in your roster, leaving only AOE Saber, AOE Rider, AOE Caster, and Extra classes vacant. Spishtar plus Skadi / Castoria will eventually fill most AOE roles pretty well, so that’s not a big concern.