Why do you think adding concrete to increase mass, in effort to reduce vibrations will not be worth the investment? (I am not sure either until I try it, just curious)
Well, the effector of my Deltas is ~0.13kg. With 200m/s travels and accel values of ~3000-5000 mm/s² i get usually 0.2g of vibrations. This is nothing in comparion to my Ender-5. Movable x-axis probably 2 kg. Additionally, the case of the ender is already way less than the weight of my Deltas. I don't underestimate if I say, that the whole Ender is shaking if I set acceleration to 3000 mm/s². Haven't taken a measurement, but but acceleration is probably in the order of 1-2 g, maybe more. Print quality on both machines is decent, beside stringing issues (Deltas with 200 mm/s travel and 60 mm/s print mainly because the BMGs become less reliable to deliver filament at 0.25 mm layer heights). Also I think it is just more economical to reduce weight of the effector than increasing weight of the frame.
please post some print pics