That was my first thought as to the best approach, but I'm wary for a few reasons:
The right 3x2 (in my diagram above) goes under/into the fireplace structure in the corner of the room, and the trimmers around the hearth are being held up by it. So a) I'm not sure I can remove it and replace...
Working my way through renovating a Victorian terraced house atm, and have progressively discovered issues with the floors in every single room. Nothing too crazy so far with most of the joists - replacing some rotten ones here, jacking up and sistering some sagging ones there - but the final...