I would welcome suggestions on how to do it more efficiently without ballooning costs.
That isn't sarcasm. If there's something I've missed I would appreciate it.
Just FYI the 4 core switches will all be running STP so will be a loop free environment despite the multiple paths. The secondary router will be using SVIs rather than physical ports for everything so could bridge the two core switches in the unlikely event their DAC fails.
The only thing lacking resilience will be the living room. This could be trivially accomplished via another wireless bridge.
Power is an 'issue' however I have solutions for that, too.
The diagram is the end result of how the network will end up. Components will be added incrementally.
February - single 330 Mb connection - All Gigabit Ethernet. ONT <> Cupboard switch <> Office and Lounge switches <> Netgear R9000 with GigE uplink to Office switch. Cost - Zero, I have all the kit.
March - 2 of the 10G switches go in place, one replacing the single cupboard switch, another the office. 10GBase-T SFP+ x 3 - not enough ports available on single switch without extra SFP+, may as well get 10G as will need later and can run in GigE for now.
April - Single gigabit broadband connection. Server gets its 2 x SFP+ NIC. 3 x SFP+ DACs, pfSense installation, Netgear to Access Point mode only.
May - Second gigabit broadband connection. Second 10G switch under stairs. 1 x SFP+ DAC to cross-connect.
Whenever, it's not important: Complete resiliency, capacity upgrade to lounge. Install lounge 10G switch. 1 x 10GBase-T SFP+. Install 2 x 1000Base-T SFP+ into cupboard switches. Install hEX backup router. Install wireless bridges.
Install UPS in both office and under stairs - to connect kit under stairs and wireless AP / bridge / switches in office but not the server.
Once all of this is done I'm good for the foreseeable. At some point perhaps a second Ethernet run to each of lounge and office to allow for core switch resilience will be run, retiring the wireless bridge solution, but no rush to attack the walls. Hopefully by then a decent 10G router will be available that isn't a server and that can go under the stairs too - where it should be!