Mate, sorry if my post wasn't clear enough, but I posted it after I already bought and installed it.
It works perfectly fine.
You are right that almost no card will fit, especially not the ones your tried, which is why I posted this to begin with.
The case will only accept cards that are 150mm< long and 68mm< tall
Details: http://psref.lenovo.com/syspool/Sys/PDF/ThinkCentre/ThinkCentre%20M910%20SFF/M910%20SFF.pdf
The Gigabyte GT1030 fits these limits and works.
I was initially going to buy the QUADRO P1000, which seems to be 150mm long, but when looking at the photos, I was concerned that the fan curve will make it a few mm too long for the case:
https://img.advice.co.th/images_nas/pic_product4/A0101296/A0101296OK_BIG_5.jpg