Hello Eric1308
The good news first, your GTX 980 Ti fits into the Sonnet case.
Until recently I had the Sonnet 550 case with the Vega 64 (which is 2-3cm longer) ->
ASUS ROG Strix RX Vega 64 O8G (8GB, High End) in use. The box also supports a variety of brands including NVIDIA (
https://www.sonnettech.com/product... - your 980 Ti is listed at the bottom).
I have to note, however, that I had to remove the built-in power supply without further ado (4 or 5 screws, plug & play). So I was able to insert and install the Vega 64 from the side - but it wasn't witchcraft.
Regarding eGPU NVIDIA and Mac -> Apple officially only offers AMD support, but there are a number of instructions/scripts for Mac to also drive NVIDIA cards-> Google will help you find the solution (I assume Digitec does not allow such "hacks" to be posted).