1 GHz RF bandpass filter

You can ignore HF coupling all over the circuit, if you know that one particular spot in cicruit is immune. Build a fully closed calorimetric metal box/chamber (2-3 cm side) with single feedthrough cap, and load it with ordinary filter and thermistor. After this calibrate the thermistor using known sources and second identical thermistor for ambient.

You will have the lowest parts count true RMS calibrated sensor. If you need higher precision (few tens/hundreds ppm) then you can employ the air-flow, oil-immersed bath, bath in vacuum, thermocouples, platinum black bodies etc.

The trouble with discrete parts is that they come with parasitics; e.g. an inductor will have a parasitic resistance, parasitic capacitance, and parasitic capacitance to the board. Generally speaking, as you approach 1GHz, parasitics start affecting your frequency response, even on small SMD components. Even if you buy "10 GHz" components, they'll have parasitics. The hard part of RF design is modeling the parasitics accurately (and knowing where they can be ignored). Also, there's a trade-off between precision and cost.

Microstrip elements are more abstract, so you generally have more control over what's going on. As others have mentioned, though, they are bigger.

Modeling components is time-consuming. Keep in mind, though, for a one-off project, you might find that tweaking the circuit is enough to get it to work. Repeatability over hundreds or thousands of units is a different matter. For this spectrum analyzer project, you don't need to worry about the 10.7 MHz IF blocks, but you will have to be careful with the 1-2GHz stuff. The bandpass filter looks like the most difficult part.