Here is what I will tell you. From all my experience with CP vpn tunnels, I learned some important things that I believe should be taken into considerations.
First off, not sure how long you been around CP, but in the old days of Check Point, and Im talking probably R60 and before, CP would ALWAYS try to present largest possible subnet to the peer, no matter if it was explicitly configured to say send /29, it would always try to send /24 or larger subnet. Thats not so much issue these days, but just to be on the safe side, I would verify below settings in Gudbedit are set to FALSE
ike_enable_supernet
ike_p2_enable_supernet_from_R80.20
ike_use_largest_possible_subnets
Now, onto main thing I wanted to discuss. That setting, one vpn tunnel per gateway pair, personally, I would ONLY use that if its permanent route based tunnel with Say Azure or AWS, but if its regular domain based, do not use that setting, unless its combination of subnets/hosts in your CP enc. domain.
Keep in mind as well, with Fortigate or PAN, makes no difference if you select 0.0.0.0/0 phase 2 selectors, it really comes down to what you have defined in the policy itself for VPN traffic.
Hope that helps, but if you need more explanation, we can do the call/remote session. Let me know.
Cheers,
Andy