Trying to understand HA cluster interfaces

Hi y'all, I've been tasked with basically rebuilding our current network from scratch, and I'm trying my best to figure out how to do this while minimizing points of failure. Our organization does critical-to-life work so downtime has to be kept to an absolute minimum. Here's a diagram of the theoretical physical topology I plan on implementing.

https://imgur.com/a/dcS6pHV#3i1TnC

Basically, I'm trying to figure out how to go about configuring the interfaces downstream from the HA pairs (running a+p). My understanding is that there's one virtual MAC address that's only active on the primary's interfaces, and when there's a device failure, the secondary becomes the primary and it takes over the VMAC addresses.

So if that's the case, how would I configure the interfaces on the juniper and cisco devices? I'm not asking about actual configs, moreso just the idea. It seems like most people discussing this topic are using VLANs and trunking, but given that OSPF w/BFD can achieve sub-second convergence times, I'd really rather use that, if possible. However it would probably be pretty complicated. Either way, there seems to be two ways to go about this, mainly involving varying amounts of LACP.

1. A single LACP bundle on both devices with a /31 P2P network between them. On the C9200L stack, for example, there would be 1 LACP bundle comprising all 6 links. Only half of them would be active, since lacp-ha-secondary would need to be disabled. Then LACP renegotiation must happen, taking 3 seconds at minimum.

2. A single LACP bundle on the FortiGates, with lacp-ha-secondary enabled, connecting to two LACP bundles on the C9200L stack. There would need to be 3 IPs, 1 for the interface in on the FortiGates, and 1 for each LACP bundle on the switch. On failover, the IP transfers to the other LACP bundle, then OSPF reconvergence must happen, sped up by BFD.

Is my understanding correct, or am I missing something? Thanks in advance!!