XY-gantry (to) rigid

I have a nicely built RR VCore3.1-300. It could run a bit better however. Let me explain: If I mount the X-gantry to the Y-linear rails with the bolts slightly loose, the gantry (without the belts) runs super smooth to the front and the back. If I then tram the gantry to the frame and I tighten the screws and/or the bolts (connecting the XY-joiners to the Y-linear rails, see pic one and two) it almost feels like the carriages of the linear rails start to bind up. It feels like the Y-rails are slightly under an angle (see third pic, highly exaugurated). I did level out my whole printer and put a digital angle meter (protractor?) on it. I could find that the mounting plates have 0.1 degree variance to horizontal (this should not matter right? not taking into acount that a digital tool of 30 EUR can have this precision anyhow). I did try to correct this by loosening some frame bolts, rotating an tightening the 3030 extrusions again but this retightening brings back this same minor 0.1 degr. variation (on both the left and right side). And now to my question(s). - Do these bolts and screws need to be so tight? - It is all designed to be rigid mounted, and as such Statically indeterminate (Statisch onbepaald,... not sure if this translated to English as such). - What would happen if the mounting points of the X-axis where a hinge or a ball joint? would the printer then be sloppy and unprecise? - Are others also having this issue and what solutions are there? I now have to fuss about and use small pieces of paper between two screw (for instance the two of picture 4) and play around with tightness until it feels "OK" ("OK" is not the same as "good", b.t.w.). Looking forward to all your ideas and feedback.