Problems I ran into
Used printers can hide multiple small issues at once. A print problem might come from the slicer, bed leveling, extrusion, belts, frame alignment, material, or firmware/control workflow.
Improving the printer required separating actual mechanical problems from settings problems instead of randomly changing everything at once.
The interface/control side mattered more than expected because poor workflow makes iteration slower even when the printer hardware is functional.
Cheap restoration only works if the upgrades are targeted. Otherwise, it is easy to spend enough money that buying a better printer would have been more rational.