I use this principle regularly, recorders + flexs can become incredible fx!
With @Kacper Fx1 Reverb hack, you can add a modulated reverb or reverb + additionnal delay!
Reverb on FX1 slot hack
A mixer is interesting for eq, but feedback (over unity) can be handled by AMP VOL, so you don’t need a mixer to try it. 2 small jacks suffice.
I need to buy different ones, needed to invert them!
For constant pitch up (1 octave), you can use 2 recorders and plock their recordings on the Flex!