Enforcing IKE/ESP IPSec Local-Out Traffic to Follow SD-WAN Rules

TL;DR: Is there a way to make IPSec tunnels configured on loopback interfaces have their local-out traffic egress out of specified WAN Interfaces using SD-WAN, or any other method?
Ideally with a preference for one Interface, with the capability to use a secondary when the primary is unusable.

Context:

Currently, we have 2x FortiGates, one 400F and one 100F (running 7.4.12), each connected to a separate 10G circuit, both supplied by the same ISP. 100F connects to WAN01 and 400F connects to WAN02.
Due to the nature of our business, we have purchased multiple Public IP blocks from the ISP, for use with various services. Some public IP blocks are routed across WAN01, and others are routed to WAN02. Currently, we simply have a static route pointing to the ISP next-hop on each firewall, and the ISP statically routes our public blocks to us via our WAN interface IPs.

I am planning with our ISP to enable BGP on both circuits, with us advertising prefixes with prepending, so that in the event that one circuit fails all our public blocks will be available over a single circuit.

The plan is to ditch the 100F and instead have 2x 400Fs in HA active/passive, with both circuits connected there, moving all 100F config onto the 400F and having 2x BGP sessions established along with SD-WAN traffic steering.

We will retain a strict mapping of 'local subnet -> specific WAN egress' using SD-WAN manual strategy rules, only failing over in the event of a failed health check for one of the members.

My question relates to the IPSec tunnels we already have established on both devices.
As part of migration, I would like to have all IPSec tunnels terminate on one of two loopback interfaces.
Loopback01 will be configured with a /32 public IP from a /28 block routed primarily via WAN01
Loopback02 will be configured with a /32 public IP from a /27 block routed primarily via WAN02.
The reason I want this is so that tunnels remain reachable upon a circuit failure, as those blocks will be re-routed to the alternative circuit via BGP prepended routes.

However, it's come to my attention that local-out traffic will not follow SD-WAN rules. I know there is setting to enable this for other local-out traffic such as fortiguard, DNS etc., but I don't see a way to achieve this with IPSec.

Does anyone have advice on how to achieve this?