Best way to connect four switch to a firewall

Hello,

Suppose there would be a network with 4 1G stackable switches (SG350X), one firewall (FortiGate 60E) and one router (ISP-provided). Router would be in bridge mode so the firewall would take care of routing.

I'd like to know the best way to connect the 4 switches to the firewall. They would share the same vlans and would also have the same amount of traffic. 99% of traffic would be HTTPS to the Internet.

I thought of making two stacks, each stack would have two switches (since there would be two switches on each floor).

Then I would connect one stack to the other, and then connect one of the stacks to the firewall.

The way I plan on doing this is:

The stack that would be connected to the firewall would have two interfaces going from each switch of the stack to a LACP interface (trunk1) in the firewall, which would have 4 aggregated interfaces.

Then, I would connect the stacks between each other (trunk2) using two interfaces for each, with LACP as well (8 total aggregated ports).

Optionally, I would also connect the other stack to the firewall the same way I connected the other (except one interface per switch instead of two) for extra redundancy (trunk3), but I don't know yet how to make it so if trunk1 fails, trunk3 would take over, since I suppose trunk3 would have to be a different LACP interface.

Does this look good or is there a simpler and / or better way to accomplish this?

Thank you!