Halsin feels a little overpowered as is he currently.
We can thank the whole Archdruid thing for that.
Its not on the level of getting Elminster as a comrade but I bet "Elly" too is not a normal Wizard anymore. Being the chosen one of Mystra & all.
And this whole "Arch" thing feels like lvl.8 at least.
Not that I am that familiar with any D&D traditions.
But for a bunch of "Mary Sue's in the making" our group feels pretty weak.
° A weak Vampire.
° A mediocre "Superwizard" with a super need for fuel
° A well known Hero of Frontiers who is also kinda like the others
° A wierd Yautja, eh I mean, Githyanki
° And the Queen of Sass & Cleric of the Goddess Edge
But more or less they all seem to be kind of special snowflakes.
Until a random Arch-something comes around the corner.
Then they are back to being just mediocre Adventurers.

Either the "NPC's" need to be nerfed, or we need to be buffed.
I recommend the first option then.
€dit:
Generally I believe we cannot have any more cooler Companions so easily.
It feels like the whole Tadpol thing is the condition for being part of the Crew.
Similar to how in DivinityOriginalSimp2 everyone shared the Fort-Joy background.
If you are not escaping from Jailtown together, you cannot end the game together?
But if I have too choose...
° Minthara - in Act.2 onwards preferably, again as a Lolth Cleric
° The Lich - our "rezz-guy" we find in the Crypt, as a Necromancer. ( God damn the crypt was cool!! )
° The "Tiefling Bard" Woman near the place where we encountered the Harpies. The one we help to finish her song.
° A Duergar maybe or the Dwarf that can be unpetrified when you slay Ethel, or use that one potion (probably).
° The Owlbear ( lol, just kidding )
° The Construct "Butler" in the Tower in the Underdark - or maybe a Spell to summon him temporarily. If he is destroyed, he must be repaired before he can be summoned again.