If it was me, (and I wish it was, I’m envious and I miss my payment implant)
I would go for a P3 or P4; P4 is where my last custom payment was, and it was a great spot for pay terminals.
I would also consider P1, but both of my P1s are taken up.
Also for me, I “couldn’t” do P2 as I have a prominent tendon in the way.
That is why you want to assess your own biology in consultation with your installer.
Everybody is different.