Totally agree about having them be the proper tension, and not overdone. On my first machine, I founn that the zip ties worked very well once I shortened them up and removed the ‘spring’ in them. Currently, my belts are tight enough that I cannot feel or see any give to them if I pull on them, but the gantry still moves smoothly and easily by hand with the motors off.
There are a LOT of variables here, and I am pretty much having to make sure I check them all, primarily due to the modifications I have made. Personally think that your original design is best for what most people are looking to do, my machine has a cut area of only 8.5"x11.5" and is intended for small mechanical parts made from harder materials such as acrylic and aluminum.