Well now… I’d hate to see you give away more of the sweat of your brow in this wretched economy! So hear me out…
Good man; that’s all I need to hear. Why not test PBR before committing it to your Slate AX? Here’s the HOW-TO I followed to get OpenWrt virtualized into Orcale VIrtualBox. The guide mentions OWRT 19.07 but the steps apply for the current release, 23.05, too. 23.05 is required for proper PBR use.
(FYI: As of v23.05 the default firewall/routing engine changed from iptables to nftables. PBR manages the routing tables for you via its GUI… & nftables is the recommended engine for proper feature support.)
A nice thing about Vbox is the ‘snapshot’ feature… so if something goes seriously sideways it’s a couple clicks to revert it.
Then decide if you want to replicate everything ‘on the metal’ using’s @solidus1983 's vanilla build for the Slate AX.
ps/ I can tell you PBR works as expected w/ eight (8) active WG client connections. 
