Electrical – How to extend the wiring so I can move a 220v subpanel

NO!

That would be a National Electrical Code violation and a dangerous practice.

That would energize the the ground wire and someone could be electrocuted from a wire they can normally expect to be safe.

It should be buried at least 18" or have at least 4" concrete cover extending 6" to either side of the conduit. Or, as mentioned, it could be attached to a wall/fence above-ground.

Pouring a 4" slab that extends 6" to either side (i.e. minimum 14" wide, centered on it) would be the straightforward solution.

Burying it 2" would be just as bad as it is now, from a code point of view.

See NEC table 300.5

If trenching rather than pouring concrete, you might want to make it a more direct run (lacking a diagram, I'm guessing it turns to run along the fence, rather than being a straight run now) in which case there would be no splices needed if the resulting run is shorter (one side of a triangle rather than two sides.) Assuming a right angle turn, you'd go from 120 feet now to about 102 feet on the diagonal, leaving you 18 feet "extra" (roughly 3 of which you would use going down to the proper trench depth.) If digging, beware of your own services (septic tank, sprinkler lines, possibly underground conduit between garage and house?) and call whatever service (dig-safe or similar) marks utility underground services in your area sufficiently in advance to have them mark any of their services. You do NOT want to dig through a utility line without having called (if you call, they mark it, and you hit one where it's not supposed to be, it's on them, not you.)