I think the prevalence of powered monitors with AES3 input means that quite a few people are in your shoes.So I'm looking for two inputs - coax and USB - and one output - AES
The Motu 8D does what you want -
https://motu.com/products/avb/8d
At US$600 it's not cheap, but it's one-third the price of an ADI-2.
If you want to do the job on lunch money, you could combine these 3 products;
1. USB-to-AES3 converter - $35
https://www.ebay.com/itm/395065071923
2. 75 Ohm to 110 Ohm digital audio transformer - $28
https://www.bhphotovideo.com/c/product/367655-REG/Canare_BCJXPTRB_BCJ_XP_TRB_75_Ohm_to.html
and switch between these two devices with a simple XLR switcher - $34
https://www.amazon.com/Balanced-Audio-Switcher-Passive-Splitter/dp/B07THV5JNS