Apologies, the 6225 was a mistake. It’s a T62100-LP-CR .
Chelsio support found a solution / workaround: pci=realloc=off in the kernel boot flags. I knew of realloc=on so I assumed off was the default and it wasn’t on my radar to try.
As soon as I added that specific flag, the driver was able to initialize properly and everything fell into place.
Now off to performance tuning and troubleshooting… 