The Gadgeteer

[Arkhios]

I don't want to rain on your parade (fencing pun not intended), but "gadgeteer" is already a word in itself.

I mean, I know that Gagdeteer is a word in itself, but the whole idea (for me) came from realization of that the word could be a portmanteau of a Gadget Musketeer (hence, the wordplay. Aaand maybe a bit of swordplay).

Also you don't need a DM being permisive with the Crossbow Expert feat, you can just take the Gunner feat.

Actually, Crossbow Expert's and Gunner's similarities end in ignoring loading properties and a hostile creature within 5 feet. Sadly, those two weren't the issue here.
The real issue is that Crossbow Expert specifically mentions "When you use the Attack action and attack with a one-handed weapon, you can use a bonus action to attack with a hand crossbow you are holding." I was implying to achieve this with a firearm instead via DM leniency.

Now, subclass-wise, if you want to be a high DEX musketeer type who fights with gadgets, you should likely go Battle Smith. Armorer is more Iron-Man-style power armor.

Yeah, probably. It's just the Steel Defender I'm struggling with as it adds nothing to the flavor or concept, and is mostly a useless liability.

My main advice, though, is to multiclass as soon as you got what you want out of Artificer. I would either go Rogue or Fighter.

In that case, I might as well take the Artillerist, as suggested above (twice), multiclass to Fighter 5+ for the Extra Attack, maybe Eldritch Knight to shore up some loss of spell slot progression, and go back to Artificer at some point.