Well, what do you mean by "efficient"? Note the significant difference between "game clicks" and "mouse clicks"! Dropping the heavy junk requires a bunch of mouse clicks, but no game clicks.
Digging XP seems affected most by level, followed by an unknown random element, followed what was dug up. Putting aside the weariness of constantly having to throw away junk, and putting aside the differences in the amount of money you could make, it may be best (in terms of xp/click) to level up until you KNOW you can beat ALL the missions, then gear up once and do them all in one blast. No lost clicks in failed attempts, minimal lost clicks in gearing up.
With all that said, I would agree that your new plan is the best balance between sanity and trying to level up with either the fewest "mouse clicks" or "game clicks". There is little point in trying and sometimes failing to collect mushrooms when it's only going to marginally increase your odds of digging up better stuff. When you can collect a mushroom that pushes you past a drop off cusp, however, that's an entirely different story. After all, once you have a ridiculous amount of silver in the bank, reaching those drop off points is about the only thing that can keep you going.
Sandy