Counterspell is cast as a reaction, so you can cast it normally after casting shield of faith.
That is also why you can coutnerspell a counterspell (despite having obviously cast a different spell in the first place)
Actually, you cannot do the former. Per 5e rules: If you cast a bonus action spell, "You can’t cast another spell during the same turn, except for a cantrip with a Casting Time of 1 Action." Even though counterspell is a reaction, it still is a non-cantrip cast during the same turn.
Meteor Swarm (9th level action-cost spell) + Counterspell? Fine
Shield of Faith (1st level bonus action spell) + Counterspell? Against the rules.
And fighter levels for action surge slow down your spell progression, that is a pretty high cost, and sure, for that, you can sometimes cast two spells, but you only have one action surge per short rest, so not reliably, and not always. It has pros and cons.
Allowing everybody to cast multiple leveled spells in a turn just buffs casters, and they already are stronger than martials in 5e.
There are two different casting situations:
1.) Being able to cast two *leveled, action-cost* spells during a turn
2.) Being able to cast two spells, one action-cost and one bonus action-cost, during a turn.
Obviously casters shouldn't be able to do the former without taking levels in fighter, which as you say has a high cost. (Related: potions of speed/haste in BG3 should not allow the casting of an additional spell.) But the latter? It's debatable how much power it really adds. The spells still use slots, and bonus action spells generally aren't too powerful, so it's arguably still balanced - slightly more burst at the cost of an additional lost spell slot.
HOWEVER, in BG3 there are effectively no rest restrictions, so casters don't have to worry as much about preserving slots throughout the Adventuring Day. This mitigates the "slot cost" of casting an additional BA spell each turn, making it more of a pure buff.
