My suggestion would be not to trade him. He has looked horrible at times recently but still seems to be able to salvage a 70 - I seem to recall he has a habit of this, popping up in junk time and/or accumulating pressure acts that we don’t really “see” in the stats. Someone posted that he had c. 65 in a recent match - I would have thought that’s a big number for any kind of act, and he seems to still be getting CBAs. Still has a game in hand vs almost every other player, and is M-F, so could be used as a M9/F7 if all goes well for you. DPP can also be handy in its own right through the byes (getting players in the right configuration to maximize numbers on field).
If you’re sure you want to trade, I’d say Bont, the Eagles and Parker are the best options above. Brayshaw seems highly unlikely to keep it up in my view, and Viney may not even be an upgrade if he and Smith revert to prior form. Cigs and Yeo also worth considering.
If you are prepared to wait a couple of weeks to get more info, some of your trade in options may then be past their bye, and there are likely to be cheaper and/or better value mids coming through (maybe Duncan, Selwood, Titch, even some of the names you mention like Gaff).
I assume you already have B Smith, otherwise definitely get him!