Maybe an idea in general:

Level 1: multitarget spells are a no go, unless they can all hit the same single target (using a singular arrow to do so)
Level 14: you can do an additional attack+spell, but this one is limited in uses (allowing you to go a bit nova, but not all the time)
Level 18: multi target spells are possible, but doing so uses up all your attacks for that action, no matter how many you can make/how many targets there are

And yeah, the level one damage remains a bit... dicey.

Think normal magic missile = 3d4 (no miss chance) or normal bow which is 1d8+mod (which is missable)
With a single arrow (as suggested above) = 3d4+1d8+mod (which is missable)
With 3 arrows (as the original writing) = 3d4+3d8+mod (with 3 hit/miss chances)

The second option is already a boost, but with a cost (spell slot or becoming missable). The third option is kinda scary for a first level character.

Also, to limit the gloomstalker tricks (and maybe others), making the use of arcane ammunition it's own action would be best. (I'm thinking about crossbow expert attacks of opportunity and/or handbow extra attacks and such as well). That way you can't increase it with other methods. (Thus making level 14 a 2 parter - 1: extra attack as many others gain at level 5/6 and 2: option to use arcane ammunition twice, but not all that often)

Then, while I'd personally keep their most powerful spells off of the arcane ammunition, doing a level divided by 2 (round up) would indeed limit dipping as well. (that said, I'm still looking at a 9/11 arcane archer/eldritch knight that can throw any and all of their spells on their 3 attacks per round (6 with surge). That'd be freaking terrifying.

Yeah, I misinterpreted the AoE one. I read it as you could release the spell anywhere along the path of the arrow (so between you and the target). Letting things go boom on your target, I'm all for that.