Well, if you're going to change the helm and make it a magic item, why not make fixed weapons that are magic items as well? For instance, instead of a canon, you have a "fireball caster". A sort of over-sized fireball wand that can only be fired every so many rounds (to simulate loading a breach-loading canon). You could use magic missile, and any number of other bolt type magic. You could also go with small pistols that do the same. It has more versatility than a wand. Once you fire it, you have to wait for it to recharge x number of hours. Or you could go with something that has refill cartridges, where the cartridge and the pistol work in tandem to produce the magic bolt.
As for movement, you are in Spelljammer's version of space. The Ether itself moves the ship. The sails are there just for show, and to propel the ship should it actually touch down on water. It don't matter what the design of the ship is, the Ether moves the ship based on the way the helm designs the envelope around the ship. That would still require the mage to navigate the ship, but it could be added as a cantrip or something that allows the mage to adjust the trim of the envelope for speed.
You could even go all intricate and create another magic item that acts kind of like the Ether Screw in Space 1889. In that case, the helm would be the magic engine that powers the "Ether Screw" (the name might have to be changed). That would work to add a sort of steampunk element if one wanted to. But it would keep it all magic-based, which, I think, is the point.
It would make fitting out a Spelljammer ship pretty expensive, but I think that's the idea anyway.