I know there are bluetooth keyboards and even wired lightning keyboards, but I'd like to get a large mechanical keyboard and it seems the wired ones only come in USB. If I use a USB-to-lightning adapter, should that work with an iPad?
In general: Yes, as this article (and others) suggest.
But it always depends on the exact keyboard. If you can't find results on google for your exact model, you can only find out by testing it.
If you try it, I would recommend using a usb-converter with both a USB & Lightning port, mainly for this reason:
You’ll want the connection kit I linked above for one main reason – it allows you to both charge your iPad, and supply power to the keyboard you will be using. The majority of USB keyboards draw too much power for the iPad alone to supply, however having the lighting cable plugged into the adapter allows you to use the keyboard as power is supplied to it via the lighting cable.