I have set mine up in a similar way but for some reason when we watch Hayu via amazon video it somehow manages to route itself to the LTE WAN3 even though the routing policy is configured to route that specific IP through WAN1 - so far my wifes obsession for 'Real Housewives of [insert which ever country they are from here] has consumed about 400gb of data in two months... short of disconnecting it and risking divorce any suggestions!?!?

I had that problem when I had WAN 2 (a 3G/4G modem) configured as "Always On", with the failover/failback to it set in the Routing Policy. Anything directed at 8.8.8.8 would insist on going via WAN2 even though the routing policy said otherwise. Setting WAN2 to "Failover" sorted that issue. Guess that's no help to you guys though.

I don't pop in much but have a similar situation. We operate a set of holiday cottages (so 40 users) and have 4MB on copper broadband and 15MB via a wireless broadband provider (Beeline using ubiquiti kit), the wireless connection has a fair use policy of 150MB/month. I can go over but not consistently.
I route all email using ports via copper as I don't care if I don't receive an email for 14-secs due to congestion, but higher priority than IoT. I route all IoT devices (PV, Netamo, Velux, Sky downloads & a myriad of other stuff) via copper. I send all Sonos radio streams via copper as we have a radio tuner handy as a line-in feed.
Web pages (port 80 & 443 from memory) get pushed via wireless for speed.
Everything has a reserved IP address in router based on device type so it is easy to send certain devices down copper only. And port redirections for other stuff.
Generally I get 400GB down copper and 100GB down wireless by implementing this strategy.
Email doesn't need to be quick, it is not high priority as delays are seconds not hours. Web-pages are better quick. If I gamed I would send that quick route with high priority.
So yes it can be done. Reserve Its based on use class and then set routing using Its and ports.