I have dismantled it many times, I know the fault, just crap manufacture that the importer would not support.After 6 years, you own it for better or worse. I think a teardown and inspection to get eyes on what's binding, broken, or defective is the only course of action at this point.
I have played with this drill many times, what ever one does makes no improvement, if you wind the spring up so tight, after a few holes it tires one out, just crap made. It needs eccentric bushes as you recommend, thanks, I just need to bore the head. I thought H&F were better than they have been, being old and partially infirmed, a machine needs to work to my advantageI'm thinking the set screw with the nut is the problem. Some of these have flats on the end of the set screw, and depending on the way they are turned can exert pressure one way or another, think eccentric action. I would try that. If that doesn't work. You may wish to put new bronze or oilite bearings in the casting to tighten the pinion up .. that would require redrilling the existing hole larger and closer to the rack.
Or if you have a lathe you can machine the bushings yourself, and make them eccentric so that you can adjust them. you'll have to figure out a way to lock them , or use loctite. If it were me. I would strip it down and figure it out. I still use my drill press even with a mill.
Like the return on my 20 ton press? good idea, would hold it up all the time, might need longer handles? I will do it, all that bushing business is not worth the effort, which is why I did not do it, thanks for that.Looking at my RF30 clone, there is a spring connected to the bottom of the quill so it always pulls up. I looked at my Taiwan drill press and a similar spring could be fitted. A collar would have to be made and a means of attaching the upper end of the spring would need to be provided but it would eliminate the backlash.