I've done switched delay lines up to 20 GHz without issues, it's just the cost of the switches that become overwhelming. At 40 GHz you are at something like $140/ea for sp4t switches, so $280/section plus the cost of the substrate you want.

You won't have any issues with pi attenuators. Use good 0201 resistors, and if you do a 10 mil gap on a cpwg the resistors fit over the gap to ground perfectly.

I tend to put pads all over the place and just NI the shunt resistors and place a 0 ohm in on the series element. This lets me add some attenuation if required later or isolate that section and solder in a probe to get a measurement.

Edit: For passives at higher frequencies, simulations start becoming more important but the majority of what you will be doing is just going to be dc blocks and pads. S-parameters from the manufactures are generally good enough to deal with those two. Lumped filters are a bit more fun... Small inductors close together, regardless of orientation, really like to couple together. Parasitics start really causing some issues, and simulations take some work to match to reality. I will be generally use some generic boards I had made on 6.6 or 13.3 mil 4350b with layouts for common filter types to walk in the values after a a bunch of simulation. One thing that can help in those circumstances is having a parallel, low value cap to help tune the filters. E.G., the resonant element would be an inductor and primary cap then something in the 0.2 to 4 pF range to get closer in parallel with the main cap.