You're not along raymo! Me and my EQ6 have a love-hate relationship, regardless of Synscan version. The past 6 months I've been using the same scope/camera setup and despite my best efforts, the lengths of subs I can take varies wildly from one New Moon to the next...3 months ago I fluked 5 minute subs, this month I could only manage 2, and I even had to throw a bunch of those away. In the summer heat, my uncooled camera can't stand the longer subs anyhow
I've resigned myself to having to drift align. It's time consuming, but the better results I've got seem less related to the polar alignment routine and more on the attention to drift in my subject area. Once give gone through a couple of iterations of the Synscan polar alignment, I align the camera so that motion of RA and Dec follows the horizontal and vertical grid pattern on the camera live view. I'll then start taking subs of about a minute and then make an adjustment in one axis, take another sub, see if the trailing gets better or worse, then continue/counter it until the trailing is minimised. Increase the sub length gradually and adjust and repeat. Then work on the other axis. It's a bit tedious, and might take an hour or two, but I've had such varied experiences with the polar routine I'd rather just trust my eyes. It's not a proper drift alignment, but it seems to work more reliably.
Sorry I don't have any pearls of wisdom to help your specific problem, but I use a small scope with camera of no more than 3.5kg on the EQ6 and balancing isn't a problem, so it shouldn't matter how little your scope weighs you should be able to get it just a tiny bit out of balance to keep the gears meshed. It should be possible, but I understand and share your frustration!