OK, so I found a PDF of the Apple Ethernet NB Card installation guide. It says the DB-15 connector I am using is a standard Ethernet port of that type (i.e. a standard AUI port.) If it's a standard AUI port, it should just work the way I've got it, especially since the AUI cable's pinout matches that of the pinout on the DB-15 port on the card.

The ROM may prevent it from fully working on a 10baseT MAU, but I can always (once I have more money) get a 10base2 to 10baseT media adapter, but I'd really like to be inventive and find a way to make the "standard Ethernet port" on the Apple EtherTalk NB Card become fully useful by modifying the driver so that the card's firmware lock-in to ThickNet coax on the EtherTalk NB Card isn't removed, as it would be by changing the ROM chips on the card, but instead bypassed. This would not just be cheaper. but it would be far more fun.

I want to utilize the IIci as a network terminal that can take what I love from newer Macs and gets them to the System 6 space, as well as doing some retro web browsing.

At this point I don't know whether it's a card limitation (which would have to be worked around) or if it's an issue at the router end.

Thanks everybody for listening.