Fair questions.
My use will be 99% lab use.
This means that form factor is not important. A display is nice but not critical, and if the display is less than 7" it is more or less useless to me. I have not seen the 4,5" NanoVNA. I have an OK sight for my age. I am actually contemplating taking a v.2 apart, and build it into a desktop solution e.g. with a 7" display, if possible. Adding some more optional buttons could be nice, e.g. for faster selection of certain measurements/features, up/down and select functionality.
For antenna measurement use I will probably pre-configure a palm size unit, since I find it cumbersome to operate. The toggle-switch is flimsy and the touch screen can be difficult to operate with dirty or cold fingers. Thus both are tough and risky to operate in a tower or on the roof.
If the component target cost is 100 USD making a modular design may not add a lot to the basic model. A modular design may in fact reduce the cost of the basic unit, since it may be nothing but the RF and processor modules. Then users may add a display of different size, provided the resolution is supported, battery and enclosure. I.e. a bit like the Yaesu FT-817 when it comes to the myriad of accessories (tuner, stands, back-pack, power supply etc.)
Calibration standards and TDR functionality apply in both cases.
Bo
Bo, OZ2M