Don't know about Starlink but I've had tons of problems with L2TP over Three 5G. It makes my whole connection unstable if the L2TP is unstable, because the firewall is constantly bouncing.
Although I did notice pfSense was set to reset all states on loss of a WAN instead of just states for the connection that dropped, so it may have been that which was the bigger problem.
Another problem you wouldn't have though is pfSense doesn't seem to support L2TP over PPPoE, it only lets you choose real NIC interfaces in the L2TP configuration. I ended up downgrading L2TP to the Lite package, so its there if Zen have any routing issues and for general testing.