Hello, Glenn.
I am not an expert, so I’ll let those who are better qualified provide the definitive answer to your first question about which model would be best in your situation. That being said, it seems the 20X, with its 900Mb/s throughput, is adequate for a Starlink/DSL dual WAN setup. There is only one WAN Ethernet port, but because you’re using a DSL connection, you can install the VDSL flex module. This effectively makes your 20X a dual WAN router.
https://estore.peplink.com/products/exm-mini-1vdsl
Yes, you can use the Peplink router in front of your UDMP. The feature you are describing is named ‘Drop-in Mode.‘ Peplink will seamlessly and transparently handle the connections without the need to make any configuration changes to your existing hardware.
Read about it here:
https://forum.peplink.com/t/configuring-drop-in-mode
Video:
