For the sake of others reading along: I understand from your previous post that you have a T1®, Model II and B2.
I'm hoping that you can find what you want to hear by using the T1® and spare you the expense and trouble of another piece of gear.
Have you tried these Presets in the T1®
ALL of the Acoustic Guitar Presets (disregard the names, just scroll through and try them out)
All of the Electric Guitar Presets
All of the Taylor Guitar Presets
The Taylor T5 Preset I understand that your T5z is a new model, but I'm curious what you heard when you tried with the T5 Preset in the T1®.
I'm also wondering if you have explored the capabilities of the zEQ and ParaEQ.
It looks like you are trying to find something to optimize the Piezo aspect of the Taylor T5z (very cool by the way).
From what I can see on the Taylor site it has a preamp built-in to support the Piezo pickup, so the outboard Piezo specific preamp stuff you are looking at shouldn't be necessary. As a matter of interest though, the T1® channels 1,2,3 (1/4 inch) inputs are designed to take Peizo pickups even if they don't have inline preamps.
When I got my T1® I was able to retire all the gear that I used use inline with acoustic guitars with Piezo pickups - even the ones that did not have built-in preamps.
I agree with you ST, I do not know the Taylor T5, but I have four Taylors, have own Taylors for 30 years, have done > 500 gigs in the past five years (since I have been performing exclusively acoustic) with my Taylor's and my Bose L1 Compact and L1M2 and have never felt the need to boost the signal of my Taylor Expression System pickups. A call to Taylor Tech support (real human beings who play the guitar answer the phones by the way, they are the best!)might give more guidance.