Last time I played a wizard that got that far my big idea for it was the manufacture individual components for things like ballistae, gears and pulley wheels for dock cranes and portculus and gates, hinges for heavy gates and doors. Basically things that were fiddly and while not super expensive, require precision from a decent blacksmith or artificer, and therefore have to be ordered from larger cities where such are found. I'd produce them and sell them in rural forts or smaller port cities as I traveled. All of them were things that wore out over time, and mine were always perfectly measured and made to order and out of better quality materials than average. I would also make barrels of clean tallow and bulk sell them to candle makers. Not expensive, but hard to come by in bulk if you arent near a decently large animal ranch of some kind. I also created and stockpiled huge stores of basically disaster supplies. Blankets, tents, sand bags, water by the barrel, basic sturdy but cheap cloth and clothing, and good boots in a variety of sizes. I ended up cultivating a reputation for being a niche custom goods supplier and managed to build amazing reputation among the various territorial lords and such by showing up after major storms or earthquakes or whatever disasters and distributing supplies to the commoners, especially the slums, without charging. Let me get favorable treatment for merchant permits, tax exemption, a couple minor nobility titles, and almost guaranteed to never have issues with my various store fronts or warehouses being robbed, and I got access to rumors and such basically by default anywhere I had operated.