I’m not a pro by any means so take my answer for what its worth.
But as Dazz_Antoni suggested (and you said it yourself) Gavial will have a better overall healing abilities, especially with her S2 that is strong and a "life saver"as you wrote.
She might not be the strongest Healer in the game, but I really like her and she is one of my main healer (in S2 and sometime S1)
I don’t have Folinic E2 yet but I can say she is usefull too, especially her damage enemy abilities. It can be used on certain occassion and help doing that extra damage you need while healing at the same time, but of course that will not be possible in ever instance. (for example on the previous CC Perma map, the one-tile wide coridor is a good place to heal and damage at the same time with the splash)
With every Pros you will have a Cons, and therefore her cons is not beeing super strong in healing (in my oppinion), doesn’t mean she is bad, of course not, but Gavial will be better for healing purpose only.
So it really depend what you are targeting, if you already are set in term of damage, if you need a strong healer.
I didn't see you posted your already E2 healer, so this part might be useless, but ho well
One thing I would note tho’, since you seems to say you consider other option, is that Silence is really good.
It’s not the best healer (or rather I think that, but I don’t want to get it on the head with a stick so I’ll say she is just really good : p ) but her S2 basically allow you to have 2 healer for the price of one.
Honestly it charge rather fast , even more if you have someone that give SP boost, and her drone doesn’t require Unit Limit (contrary to a lot of sumon) and you can place it anywhere in the map as long as there is a tile available.
So while I won’t advocate for beeing THE healer to raise, I would say if you’re limited in ressource/time and need an healer, she will be good on a lot more occasion.
Now that beeing said, the first two are still good, and based on what you already have, Folinic might give you that versatility in damage.
But if you really want/need a strong healer then go Gavial I would say (or Silence).