Solved, it was indeed just a simple tweak. I unchecked the System -> Routing -> Miscellaneous -> Failover default gateway option I had checked, and updated any outgoing firewall rule to explicitly use the advanced settings. Under advanced settings I changed Gateway from default to the name of my Gateway group. Now, outgoing firewall rules have a black sprocket icon next to them to indicate advanced settings which in my case is the Gateway group. Internal traffic rules, any LAN -> LAN for example, do not need this updated setting. Only traffic using a Gateway should be updated to use a Gateway group.
To answer my initial questions explicitly: yes, this has to apply to each firewall rule that allows WAN traffic. If you have an outgoing firewall rule with the Advanced Settings -> Gateway left on 'default' it will NOT allow traffic to leave via the secondary WAN. I had to update my SSH rule and HTTPS rules independently for both to work. This is all clear in retrospect, but this might help someone else in the future to learn from my confusion.
Once again, thanks heper!



