I agree with his short comings with his neutral game and lack of projectiles. However, I think his offensive strengths are in his pressure game and combo game. Unless out of a grab or using d-smash in an edgeguarding situation, I almost never use his smash attacks. If you're good at reading your opponent, then Kirby has a ton of tools that you can use to wreck them.
Most of the cast have smash attacks that are reliable, and as you said, you hardly ever use them. Dair does combo into smash attacks so they can be used that way for a guarantee KO, but there are many situations where the smash attack just doesn't work like one would think it would. For example if you dair against Kirby and certain other characters and then do a just frame Up smash right next to him, the attack just whiffs. It looks quite awkward.
Yes if you're good at reading your opponent then Kirby can wreck them, but that applies more or less to the whole cast. It's not a character specific attribute.
His ground mobility is pretty good and wavedashing around with him is great because he duck under many things. He also has quick options to link together combo strings with side cutter and fast-fall off the platform dash attack, both of which work very well. If you hit with the aerial hammer consistently it's devastating, plus the fact that it's land-cancelable makes it relatively safe. A lot of the time you can even combo from bair to hammer off stage when the opponent tries to combo DI to avoid another bair.
Of course I know how well his ground mobility is. I was a Melee Kirby player after all. He is small and quick (he has good acceleration it seems), but doesn't have great ground top speed (27th/41). Many characters have better mobility than him overall on top of having better range or other better things. If Kirby had good air mobility (37th/41), then it would make more sense, but he doesnt.
All the offensive options you mentioned here are as you said. Aerial hammer is good, but I do feel that if any one of those hindering factors were a bit better, it would be more reliable on the whole cast. Although I am relatively consistent with it, I feel that it may not be reliable in top level due to all the hindering factors.
His throws are one of the most important parts of his game. They may not kill on their own, but they're designed to set up into other moves that kill. His up throw works as a DI trap and sets up great options to kill off the top with an up-air (the move I probably kill with the most and even combos out of dash attack) or a side cutter into another aerial. His forward throw is another DI mix up that yields good results. If the opponent DIs inward, you can start a juggle chain with up-air. If the opponent DIs away, you can pursue with side cutter into another aerial. Down throw is a notable good tech chase option on those with higher fall speed, but the true power of it lies in the floaty matchups. Kirby has a true chaingrab (with JC grabs) on most of the floaty cast at relatively low percentages, and can finish them to death with a up-smash or f-smash. His back throw probably has the least utility and should only really be used to set up for edgeguard opportunities (although at low percents, it's pretty easy to follow up with a b-reverse side cutter).
As a Melee Kirby player I recognize that his grab game is extremely important. His Up throw can always be DIed away by most characters. Forward throw does work as you say, but at higher percents Kirby can't pursue with side cutter and so they get away with just the 4%. Down throw is as you say. Back throw is good because of the trajectory allowing for good edgeguard attempts, and it does a twice as much damage as forward throw guaranteed.
I don't see why Metaknight and many other characters have way more damaging throws than Kirby, on top of having equal if not better followups. I feel that Kirby should do alot of damage for getting grabs.
He may not be a power house in the same sense as Fox or Lucas, and his options largely play on being able to out think your opponent, but he has way more offensive options than people are aware of.
He does have more options than people are aware of. A lot of people don't really know about or understand the character, but I would not call him an "offensive power house" because of that.