You can’t change the router’s GW address (bypass mode is a workaround, but it’s not always acceptable)
I See no problems in adding multiple Starlink gateways or even multiple cable modems that operate in the same providers subnet and offer the same LAN subnet … If you accept SRC-NAT.
Those ISP gateways are identified by a separate local Ip address in my network. Therefore they are easily addressable as specific devices.
The MT facing the ISP gateway will do the SCR-NAT to the required subnet for the modems and gateway. From that MT onwards they can all be the same.
Cable modem provider here uses very large subnets for its (2048,4096, 8192) clients modems WAN IP addresses. So neighbors are in the same WAN subnet. Getting there via another (eg DSL) provider while load balancing could be a problem.
I did use load balancing on multiple cable modems from the same ISP, to have a muitiple of the modems bandwidth. (Yes it goes over the same ISP backbone, but that has a much larger capacity than the cable modem). The same applies to the 150Mbps Starlink connections.
The Starlink APP connects to the original Starlink-gateway or Dishy fixed IP address only. It will take the then active LAN route to access that GW or Dishy on the original IP address. That is only a potential problem for the local Starlink traffic, not for the forwarded traffic.