Just wanted to add this incase anybody else is looking to replace their mesh system. After consulting the OpenWRT forums per @Catering9655 suggestion my current plan is to use the R4S as a replacement router
I will use the WSM20 for APs
This seems to be the most viable replacement option without tripling the cost.
Also, because both have OpenWRT support I should be able to run a VPN using wireguard on the network if needed.
I have not looked into whether it meets all my initial requirements but from what users have said on this forum, it seems like they may have been to strict anyway.