question about topology

[fortinetuser2020]

a new system

3 branches + 1 main site

all fortigate - 4 units

in each branch, i have 2 p2p lines from separate ISP's

and i have 2 parallel lines on the main site

so i actually have 8 physical ports in the main site. 

1 cable from isp 1 for main branch

1 cable from isp 2 for main branch

1 cable from isp 1 for first branch

1 cable from isp 2 for first branch

and so on and so forth. 

these lines are L2 lines, no ip over them

the objective is to deliver point to point connectivity between each branch and the main site, with failover to 2nd isp in the event of a line failure.

so what's my best practice should be?

at the branch offices, i was thinking about settings up a unique ip to each p2p line, and using bgp for failover/failback.

all routing will go through p2p-1, and bgp will failover to p2p-2 in the event of a failure

so if i'm correct so far, it brings me to the main site.

what should i do there? should i set a unique network to each port? remember that i have 8 ports, 4 from each isp.

also, the isp had set the lines that each branch can only talk to the main branch, and not each other.

thank you in advance :)