

Since you mention the Ender 3, mine came bundled with a laser cutter attachment. I have never actually gotten around to using it, so I can’t offer much other than to say that such an official accessory does exist and might be less of a hassle than some of the DIY options you are considering.
True… but i mean lasers are cooler by virtue of being lasers. 😁
Personally, I’d be worried about subjecting the printer to the lateral forces and torques a knife would put on a printer that was never designed to contact the workpiece, if you ever wanted to use it for FDM printing again. But maybe that would be a good project for a printer that was already beat up a bit.