This is fine, except you miss this:
option defaultroute '0'
In luci if you edit the server interface in the advanced settings tab it reflects the use default gateway checkbox which by default is checked, and in uci not lined, by explicitly setting this to 0 it unchecks that checkbox, so this interface does not get seen as a wan type interface, otherwise this could clash with your internet wan connection, you only want wan to be the default route.
All other things look correct here.
this looks correct.
As for the firewall you may want to not put the server interface in lan, instead you want to create a new firewall zone make sure the input is set to ACCEPT and not to drop or reject.
Then you edit the zone, and add the zone to forward to, this must be wan, or wgclient depending what your intentions are, note that for wgclient you need to perform some extra action so that the fwmark is applied, if thats the case just ping me again 
now lets say you want to make the server accessible for lan but lan not visible for server, then you go inside the firewall settings where you defined the zones, there is a tab called traffic rules, please add one and ensure that the source zone is lan, (optionally to restrict further you can limit it to a lan client with source ip or source mac), and as destination zone server, then you just define under action accept.
So this way if you made first contact, the server is allowed to communicate back on the same line, but the server can't be communicating with lan if you didn't iniated contact first.
This page will help you more, and also with wifi, it is important to know you must restart the router so that the wifi works properly and applied.