I agree with you that the team requires a second marking target up forward and that a second genuine KPF(not elite just competent) is the most sustainable model but it is not the only model.
Given the composition of the list there are two possibilities that stand out for me. First is one of Cox or Reid at full forward. The problem with this is that they lack mobility. Therefore if you go down this route the forward structure has to be set up so that they always have one or more smalls in support, first to crumb off any aerial contests and second to provide defensive pressure, either directly or by corralling the opposition back towards the big guys(they may lack agility but they still take up real estate).
The second option would be to use Fasolo as an undersized key forward(I distinctly remember a game against Richmond where he made Rance look second rate in many one on one contests). Again for this to work the forward line structures have to be organised to suit, in this case that would mean coordinated leading patterns to separate the defenders otherwise an undersized forward line is liable to concede too many intercept marks.
The two obstacles to either of these plans working are first they require a level of organisation and team work among the forwards that has been absent the past several seasons. The second is that the four main forwards in these scenarios being Moore, Elliot, Fasolo and Cox/Reid are not able to provide enough at ground level. Therefore as the two lesser talents of the four only one of Cox/Reid or Fasolo should play in the same line up and the other replaced by a small who can provide defensive pressure while still contributing on the score board.
That leads me to my conclusion that if looking for a ready made player to improve the forward line it is not a key forward but a small that should be targeted while another tall is acquired through the draft. A quality ready made small will be cheaper allowing pick six to be retained hopefully for use on a young KPP while also providing flexibility to the coaches when determining their preferred forward structure by compensating for what would otherwise be top heavy set ups.
All of this is true. The aditional factors are well known too. The modern team defense that we find so hard to penetrate changes what is actually required on the ground. Players like Betts are rare, but that is what we need to balance our forward line. Elliott occupies this space, and is a top player, but doesn't provide enough ground level devil. He is too good to consider moving him on or to a different part of the ground. Real pace, goal sense and foot of the marking pack ability is what we have to add. All in one player.
Watts is not that player, but he would add teriffic kicking to the team, so he has to be considered. He could be a link player in the mold of Lynch, but his failure in the grand final should be noted when looking to rely on this kind of player under pressure.
Adding Watts pretty well consigns Mayne to the scrap heap, unless a new role is found, and we have a lot of medium defenders. He may be good as a lock down defender. There is nowhere for him in the midfield.
I would add that it is folly to expect that all of the players one might want on the field will be available any given week. Backup is required everywhere, and one of the consequences of interchange is that spare parts players have a place. One of the few good consequences of interchange.