Most articles talk about the TRL obtains the most accurate results at the higher frequencies. I find it odd they offer standard SOLT kits rated for 10+GHz.
The point of using TRL with the test board is not precision, but to place the reference planes on the PCB and to calibrate the coaxial-to-coplanar waveguide transition out. This is the reason why TRL is usually the method of choice when a calibration in a planar geometry is desired. For precision measurements at higher frequencies one would, however, not use connectors, but a probe. This also works on wafers, and is used for on-wafer VNA measurements in MMICs.
You can also do TRL in coaxial geometries using airlines. This is, for example, done in metrological contexts since airlines can be manufactured to extremely tight tolerances. Moreover, in this way scattering parameters can be traced to dimensional standards, and that's what national metrology labs use TRL for. However, airlines are delicate and tedious to handle, and very easy to break. So you don't want to do TRL when you only need to calibrate to a coaxial connector unless extreme accuracy is required at high frequencies. In almost all cases you simply want a good quality SOLT/TOSM kit.
Another feature of TRL: Cal standards do not have to be fully known (as opposed to OSM). For example, the line can have an arbitrary transmission coefficient (which includes loss); only the wave impedance matters. Similarly, the reflect standard can have (in theory) an arbitrary nonzero reflection coefficient (but which is assumed to be equal at each port). In fact, the TRL mathematics allows to obtain the transmission coefficient as well as the reflection coefficient from the underlying system of equations. But normally VNA firmware doesn't output them, but this could be done offline with your own software written in e.g., MATLAB or Octave. This is sometimes done in a two-tier calibration to measure wave impedance in planar geometries.
There is a frequency restriction due to line length: The phase shift of the line must be kept away from 0° and 180°; for wider bandwidths several lines have to be used. Extension to low frequencies (in fact, to DC) require an additional match (this is then called TRM). Modern VNAs automatically take care of that.
See the writeup here for some more details:
https://www.mariohellmich.de/projects/trl-cal/trl-cal.html, and also the quoted literature.
They also offer some practical guidelines, like using precision connectors.
Yes, the test board shown above depends on the equality and repeatability of the connectors. Since the reference planes are on the PCB, the assumption is that the path from the VNA port to the reference plane is equal for each cal standard, as well as for the DUT connection. And this includes the connectors and the connector-to-coplanar waveguide transition.
I don't think that with my test board I have too much of a repeatability issue (more a feeling than a fact) and results are fairly reproducible, but that's only up to 1.8 GHz with half decent connectors (about €8 each). For more precision and/or higher frequencies you will have to invest in 3.5mm screw-on launchers (e.g. from Huber-Suhner), be prepared to pay at least €150 each. The board dimensions must also be sufficiently accurate.
Before I go down this rabbit hole, could you have a look at that spreadsheet and give your thoughts.
Sorry, I have a ban on Microsoft Office here and can't take a look at it. But you only have to observe the phase shift restrictions with the line standards, and when using two lines allow for sufficient overlap. You can also do the math with paper and pencil. Or start with just a single line when you don't need a wide frequency band.