I have a i350T4V2 nic in my extended Wyse. It's what i use for the 'important' interfaces, like WAN and LAN. My Wyse has 2 realtek IF's, the one integrated, plus another one a original second IF.
I just got a new minipc, a GMKtec K8 Plus. It comes with 2x intel i226-v 2.5Gbit IF's.
I have it hooked up now to one of the Wyse integrated realtek's. At first, i had forgot about the intel 2.5gbit issues. I booted a debian live image on the K8P, i noticed that something was wrong, and to my horror, the link had been established at 100 mbit.
I went into my opnsense settings, and found that i had long ago disabled hardware offloading on my realtek interfaces, for the reason the OP writes, because i had heard horror stories... I removed the checkbox in opensense that enables the 'override global settings'. and disables the hardware checksum offload etc etc..
But soon after, i remembered reading about the intel issues with their 225 and 226 series. (link dropping, and/or being established at 100 mbit for no reason) I've disabled ASPM on the K8P. I've forced the auto-negotiation to 1000Mbit on the K8P Now the link is 1 gbit, but for example in opnsense details on the overview of the 're0' interface i see 11 output errors.
Now, i have NO clue if it's because the wyse has old bad realtek nic's, or if it's because of intel 226 series issues. I'm lost.