Arrow of Detonation is the former "fireball arrow" from the predecessors. I guess you mean the knock back arrow:
https://baldursgate3.wiki.fextralife.com/Arrow+of+Roaring+Thunder I would correctly rename the arrows to projectiles in general because only bows use arrows. Bullets / stones are for slingshots and bolts are for crossbows. No idea why Larian calls everything arrows...
I think your approach of only creating surfaces with indirect targeting isn't so bad. Then you can deal direct damage to enemies and reduce the chance of accidentally hurting your own party members or neutral NPCs.
For a direct hit, I would slightly increase the elemental damage and for an indirect hit, reduce the AoE damage accordingly.
