Keep everything the same EXCEPT...
...bring back the insta-kill on direct hit. Meaning, keep the 3 shard hit limit, except when the initial blast detonates directly on an enemy, in which case the enemy will get hit with all the shards (makes sense, right?). Maybe extend the fuse time by a tiny bit for balance.
Not only will this bring back skill, but also some much needed DPS, which the old shard bomb line was famous for. Additionally, the damage boost will be reserved for the skilled only, and only works on single targets, so it balances out. PLUS, it won't function like a gun, which prompted the shard changes in the first place.
Thoughts? Comments?
The original shards were usefull for their range (plus ability to go over small walls) and the insta-kill was probably the reason they got nerfed.
You're doing it the wrong way.