My Stealthy worked wonders in V5 and he's probably the least niche of those champs. They can work at R4 in the matchups you need them for.
Sorcerer isn’t niche at all, being able to regen off block means not having to deal with dexing those hard to evade specials. On top of that she has the 2nd best damage a mystic champ has without suicides (not counting longshot against his ideal matchups) , slow, nullify, armor break etc.
You seriously lack a high rank Mystic. For Act 6 (idk how far are you, but I doubt you have the champion chapter 100%) you need good Mystic and whole 2nd chapter of V5 is for buffing mystics.
Sorcerer Supreme has got some nice utility and is fairly versatile.
Now I have to say that this is based on my personal experience, I have both Stealthy and Sor.Sup and I dont like Steathy too much. I also basically hate Symbiote Supreme so that's why I didnt vote for him. My adept for the most boring, rng based playstyle
Symbi obliterates a large portion of V5, is excellent against the otherwise annoying Storm and Venom bosses, and does a number on some pieces of act 6 as well. This seems like the most obvious choice.
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Sorcerer Supreme has got some nice utility and is fairly versatile.
Now I have to say that this is based on my personal experience, I have both Stealthy and Sor.Sup and I dont like Steathy too much. I also basically hate Symbiote Supreme so that's why I didnt vote for him. My adept for the most boring, rng based playstyle