I have the Starlink residential service connected to WAN2 on my MX85. Starlink has gone back and forth between using CGNAT IP addresses and Public IP addresses for the residential service. They are currently back to using the CGNAT IP Address range. They are supposedly using Public IP addresses for the Starlink business service. As long they are using CGNAT on the residential side, Client VPN will not work across that link. Site-to-site VPN will continue to work. Starlink has also experimented with handing out IPV6 addresses too, but they are not currently doing that on the residential service either. Your best bet is get your Optus service repaired, if possible, and use it for Client VPN. Your second choice, if you are located in the US, is to switch to the Starlink business service, which is also more expensive. The residential Starlink service also has a low (< 2%) continuous packet loss. I have attached a couple of graphs depicting this.
Insight WAN HealthUplink Latency and Loss