Mayfield is a shot blocking and hitting machine, and has good underlying analytics defensively. He enjoyed a career year, yes, but if Mayfield slots in a 2nd pairing as the compliment to Power, then Mayfield doesn't need to play 20m or more every night even though I think he's capable of it. Not too many teams have 4 dmen who all play 21m or more a night; Dahlin Samuelsson and Power will be the main minute munchers no matter who you get to slot into the Top4 alongside them.
FA and Trade both carry risk, but better dmen than Mayfield will cost a lot and tbh idk if the Sabres should be dishing out premium assets for a dman when there are other needs as well like in net. I don't think the Sabres need to rush anything, there's lots of talent in the roster as is with more in the pipeline.