Thanks!
Transimpedance/transresistance amp design seems to have become an axiom. It is assumed that the high open loop gain gives solid advantages-just select the desired amplifier, add a feedback resistor and the amplifier is ready for typical use - with photodiodes or something similar.
However, commercial picoammeters have a different topology and I paid attention to the Advantest picoammeter. The first stage is a buffer and it does not take part in loop gain. It allows you to achieve a giant input resistance of the device. And by the user's choice, the gain may even be a unity - it's so unlike the widespread solutions that I decided to do a theoretical analysis. This is what the calculations are made for. Now anyone can use them, the only problem is that the exact value of the input resistance also depends on the insulation resistance, for example.