Did you work it out?
I was also considering purchasing a Buttkicker pro. I had a Buttkicker years ago, and used it with xBox 360, Playseat and Forza 4-5. Sold it when I moved and was disappointed learning about the PS5 audio limitations. Hesitating on buying again.
Read about Simhub, but itʻs not clear to me it will extract more than "sound" from GT7, and the setup looks messy, Buttkicker has alot of wires and youʻll also need a laptop to run Simhub and cable that to the Buttkicker.
From what I read on the forum, a HDMI splitter from the PS5 to the Buttkicker can be used, but is just "sound", which I understand to be the audio output from GT7. So that might mean engine sounds and road noise would be mixed with music and menu sounds.