Ethernet seems far easier to prototype with. There's almost no off the shelf stuff for talking to RGMII whereas Ethernet you can just plug into your laptop for testing. If it's two different teams building things it seems like it would be a lot easier to just agree on Ethernet as the interface and then delay integration testing or release earlier.

RGMII isn't really designed to go board-to-board, fairly high data rates, and ideally all of the signals should be delay matched. That gets a bit trickier when there are two boards involved. Also I would expect EMI/EMC issues.

I know people do that sort of thing for evaluation kits, but it doesn't seem like a good idea for production.

A lot of this is pretty POC-y. Agree digital to analog to analog to digital is kinda inefficient, and in the abstract MAC to PHY (which is probably what you mean when you say MAC to MAC) with RGMII is probably better. My off the cuff guess is that it is likely the written-up interface is easier to access or requires less diving into internals. Not sure where the RGMII lines are, and depending on the design of the Starlink mini itself (I am ignorant of this) the lines might have been buried deeper and less accessible, who knows.

I'm not sure I understand the entire point of the exercise. There's already an RJ45 jack on the Mini, so no need to hack the unit to get access to an Ethernet PHY. And the WiFi router can be turned off via the setup page.

Did they remove support for the Ethernet jack on the Minis available in Ukraine? It looks like it's still present on the WiFi board, next to the power jack.

That connector is way cheaper than something that could carry an RGMII signal without major reflections. It's probably cheaper in the end to have the extra silicon than a better connector, and I don't think you need the magnetics if you don't need isolation.

It could just be that using Ethernet allows for more flexibility in what is connected to that board-to-board connector. In this instance, it's a wifi router but a customer may instead want a bare port to connect to their existing infrastructure (just the modem). It also means you don't have to worry about which MII standard both boards use, they can be upgraded independent of each other. The physical interface for Ethernet remains the same whereas MII has different pin counts.

Sure, then you get to write an MDIO emulator so that it actually detects link and since it's a proprietary system you can't exactly comment out the SMI code.. fun!