I have built three very high input impedance amplifiers; under 1 pF, over 100 MΩ.
One some else's design and layout, very high input impedance, but low frequency; several MHz. Not shown below.
The second one I layed out hoping frequency response to 30 MHz, but it dropped off of 17 MHz. Probably poor layout.
For the next one I received advice to skip the SMD and use through-hole (seems counter intuitive).
It is a different schematic and is supposed to reach 90 MHz, it did pretty well, but had a level bump at 29 MHz, I had etched away the underside ground plane, I covered it with a grounded copper tape and that removed the level bump and it is pretty flat to 30 MHz, well under 1 dB.
Now I want smaller, so I'm going through the learning curve of learning KiCad to design an SMD high input impedance PCB.
Any hints on how to limit the strays that cause problems?