Here follows what I did to the Genmitsu 3018-PRO CNC Router (Now only $199)  to achieve the precision I needed:

First the result, PCB 44 x 32 mm with 0.6 mm drilled holes, the drill bit lying on top for comparison is a 1 mm diameter:

Tweaks done to the Y axes table:

- to take out the play of the table on the Y rods and still maintain the 90 degree angle to the X axes, the four linear bearings and the rods themselves were carefully adjusted to minimize the overall play in the table. This implied that linear bearings had to be turned a little off the rod direction to get a tight fit. It was a long process... To take out the backlash in the Y axes screw and nut I drilled M4-threaded holes in the nut and put in M4 screws to eliminate the play, since the anti-backlash spring was too weak. The orange part is a plastic piece to avoid rotation from vibrations.

-The result: No Y axes play or backlash at the forces needed for PCB production, but then the Y axes stepper motor was too weak when microstepping.

Tweaks done to the X axes table:

- Since the X axes linear bearings in the Z axes assembly could not be angled to remove the play on the X axes rods, I decided to design and 3D-print external 'bearings' that would fit tight on the X axes rods. They were attached to the Z axes assembly with M4 treaded rods and nuts as shown in the photos. Then the screw backlash was fixed the same way as the Y axes. Finally, to eliminate the springiness of the X axes rods themselves, four wooden clamps (red dot on one of them) were put on the X axes rods. They are adjusted to the needed width of the actual job and then clamped to the horizontal beams behind them.

-The result: No X axes play or backlash at the forces needed for PCB production, but then the X axes stepper motor was too weak when microstepping.

Tweaks done to the Z axes table:

-there was no play between the Z axes rods and the linear bearings that needed adjustment, but the screw backlash was fixed the same way as the Y axes.

-The result: No Z axes play or backlash at the forces needed for PCB production, but then the Z axes stepper motor was too weak when microstepping.

-So, new stepper motors were needed. I found a kit of three at Amazon at approximately 50 euros. The photo below shows the difference between the original motor placed on top of the new motor for comparison. I mounted them and ran my usual test PCB card, the result is shown at the top of this mail. So I have finally made my Genmitsu CNC Router do what I wanted.

I attach the Stabilizer STL file for the X axis rod 'stabilizer', and below is the setting for CURA slicer to get the piece sturdy enough.

The rod hole in the piece have to be adjusted with a round file to to fit tightly on the rods. Do not overfile!