There's a cliche of three things you shouldn't mess with when house ruling/home brewing in 5E: Attunements, Action Economy, Concentration. Combos like this work because BG3 messes with all of the above to differing extents. In fairness there is official content that slightly effects each but it's much more measured than BG3.
The implementation of haste and plentiful availability of potion of speed alone is fairly busted. Even without using a lot of spell slots, two lots of double attack Eldritch Blast at level 5 with Hex and Agonising Blast would be almost 2/3 the damage of three Scorching Rays, and sustainable turn after turn. It takes one turn to fully boot up with current items, unless you replace one of the Eldritch blasts on the first turn with a fire spell... or there's a way to apply fire damage to hex I'm not aware of (wouldn't be a huge surprise).
But I don't know if putting that in players hands is too bad. For example enemies can extra attack to chuck multiple bombs, particularly annoying when I've stuck disadvantage to their attack and they miss twice, but still deal almost full health damage and force three concentration saves (two for the AoE damage, one for the surface). Either BG3 needs a root and branch rework to bring its combat closer in line with 5e's model of balance (which is far from perfect but IMO better) or more likely at this point, these kind of shenanigans are just what BG3's combat is.
