Baldur's Gate 3

I'm doing two simultaneous runs, a good Sorcadin run on Tactician and a dark urge, evil Berserker run in Honor Mode that follows along. The good party is basically acting as an advance team to remind me of the scary encounters, ambushes etc. but I've also never done an evil durge run so it's not exactly a 1:1 comparison.

The evil party is just shy of Act 2, but I'm playing with the only companions I have access to (because of my evil actions) and I'm roughly trying to play them on type. Astarion is a Gloomstalker Assassin, Lae'zel is an Eldritch Knight and Shadowheart is a Light Domain Cleric. We are cake walking at the moment, didn't really break a sweat through the Underdark and Monastery. I'm concerned at not having a true caster... But I'm also sort of intrigued at the viability of this party.

Counterspell is the big thing I'm worried about. The advance team is proccing it constantly. Also replacing Haste with potions would suck and I can't get the Haste bow (I assume, Dammon is dead). So far relying on cleric skills for AoE has been working okay.

Can I get away with using speed potions and maybe something clever like going full anti-magic and using Sussur blooms with some party tweaks?(edit: apparently that's not possible anymore, but still Silencing weapons etc.)

Or, should I bite the bullet and spec Minthara as something castery when I get her? If so, who do I replace?

Any suggestions welcome.