@mikefromoz - thanks for the comparison and insight, this is very helpful info! I figured I would follow up as well to share my experiences with these cards. I initially bought a Chelsio T520-SO-CR in 2018, but have upgraded since to a pair of Chelsio T540-SO-CR's. I have been using these cards for about 1 - 2 years now - one of them in my pfSense firewall, the other in a Promox hypervisor. They have been rock solid - I definitely recommend them. From time to time I see them pop up in the Chelsio store - I think it's a pretty good deal for a quad port 10Gbit card:
https://store.chelsio.com/collections/current-adapters/products/t540-lp-cr-4-port-1-10gbe-low-profile-uwire-adapters-with-pci-e-x8-gen3-32kconn-sfp-connector