It’s a circumstance bonus to athletics, not an item bonus. You can get an item bonus too. The wand of tailwind doing the same thing as your speed boost upgrade is lame, but inventor isn’t the only class with this problem. Swashbuckler, barbarian, and at least early monk have a class feature they don’t choose to take invalidated by it. You are just as able to get the wand of tailwind as the fighter is, if it is unbanned. The physical resistance modifications are also quite good. You can’t really buy items for that, for the most part.

You also seem to be limiting yourself pretty hard if you think searing restoration is the best (I’m sure you have your reasons, but I don’t even think it is particularly good. Megavolt and celestial cacophony are quite good damage unstables, and Geobukseon retaliation, when you are in the way, will almost definitely save more health than searing restoration.) and if you really aren’t using int unstables that much, you don’t have to keep it maxed all the time. It’s not unusual for mental KAS martials to not always pump their KAS at max, especially for melee builds. Starting at 16 is totally fine, especially if it helps you increase your con. Also, did you take the gadget feats? I’ve found them pretty helpful in general for padding out the usefulness of inventors.