The M8 screw im using are not an perfect fit inside the 608 bearing, they have a little gap of halt mm or less. this is not much space, but in some case it can make the difference between all 608 touching or not the tube. but In your case the main problem is not having an tube with the correct 25mm… if the gap is to big, i guess the only solution is to reduce more the size and print again!
“It will be no problem to configure the firmware to give electricity to all motors continuously. That was may guess for a solution.”
This is not the reason of your circle problem. The electricity is only cut when an motor is not used for several seconds. When making an circle, all the x and y motors are always in use. This cut of electricity is mostly an problem exclusive of the z axis.
“I’m also quite sure I don’t have any missing step, first of all I checked very carefully the vref on my drivers, then I’m quite shure in case of missing step I should not obtai an ellipsis but an open curve… similarly at what happens on a 3d printer that produces slided layers.”
Most 3d printer use only one motor for the x and y axis. In this cnc they are 2 motors for each axis, what will happens if only one of the x axis motor miss steps?