First question is do you need a 10 gig NIC, and the issue among other things is B-series may at best have a TB header but it still requires a MATCHED TB card which will cost hundreds. If you want 10 gig it will be cheaper to just buy a new Mobo (you should consider Z-series) in the future as it has general better connectivity OR there are some mobo out there that have native TB3 in the mid $250+ range.
So money wise it is definitely better and for compatibility to upgrade the mobo first, I'm pretty sure you cannot use TB on this board in the config you have. You could consider a mITX which has dual 2.5 and you could bond them and use scalable CIFS if that is a reason, then of course there is upgrading your entire network apparatus. 5 Gig should be enough and if you need critical work consider direct connection first.