A question about 90°phase shifter

I'm designing a 90° phase shifter @ 20-40kHz using Pi-type network. The requirement of that circuit is that it must stably work @ 20-40kHz. To meet that requirement, I series R behind two capacitances. Now have the problem, the requirment of phase shifting is meet, but value of S21 only have 0.02.

How can I do to trade-off such problem?

RC-CR phase shifting network is has good phase accuracy but bad amplitude degradation, that's the reason behind the attenuation caused in your S21 response, use subsequent stage of amplification buffers to compensate the loss.

Rgds

Here is a paper about a network with quadrature outputs and less attenuation.