Downloading drivers is typically a windows concept and you definitely don't need to download drivers for that card with the 4.18 kernel. I have five of those cards.
About to pull the trigger on two of these cards, connecting a Debian server with OMV. Do these work out of the box in OMV4 (it sounds like you have it working)? What kind of iperf results are you seeing?
EDIT: Set up, and tested, seeing 10Gbps using iperf3.