I can confirm the Netgate 1100 runs very well with dual WAN, and failover takes but a moment. In my case FTTP (300/50) + LTE (30/10 typical but an upcoming provider swap may improve that). No issues with throughput or performance.
Given your intended deployment I see you might end up with double- (or even triple-) NAT. This may cause you issues with throughput because of fragmentation/retries due to the squeezing of the MTU size with adding so many headers.
Also, one lesson learned from having a radio-based element in the WAN setup - in my case LTE, but probably applies to Starlink, too* - dial-up the latency numbers on the gateway monitoring. Ping times may grow to over 2000ms under load so we don't want the gateway monitor to think it's down just 'cause it's busy!
(the only Starlink I have access to is in a production environment. I don't think my colleagues would be happy if I 'borrowed' it just to test a hunch)