Glad you got it resolved. I really do think the smaller holes would be fine without vents if you choose to test out more iterations of your design in the future.
@Pankl_3D underscored a good point - if you need dimensional accuracy in the Z direction (i.e. up the height of the part) then you either want to print on supports, or tweak the “Z-Compression Correction” advanced setting in Preform. As he said, the first several layers come out squished. I have a diagram illustrating that here.
If printing directly on the base you might also notice some bleed around the edges. I’ve found that can usually be reduced or eliminated by adjusting the Z Fine Tuning on the printer.
But lifting it onto supports eliminates both of those issues and your cupping in one stroke, and gives you a nice, chamfered raft to ease removal.
(Personally I like printing parts like this on the base, but it can take a bit of tweaking to dial in acceptable results).