I think you are on the right track, but if you keep your "newish 6.0m2" sail (by the way, what mast are you using in this sail?) a 7.0 m2 Tush would not give you the range (light wind planing here) that you could get with the same sail in a 7.5 m2 size.
Since you are only going to have one or 2 sails, it pays to spread them out about 1.5 m2 (above 6.0 m2 anyway) as this covers a much broader range of conditions and there is less "overlap".
Get a 7.5 m2 Tush T-Bird with a 75% or higher (Tush FreeRace 80 would be good) and you would have a really solid rig for up to 18 knots (with some tuning).
If you can only have one sail, then your plan for the 7.0 Tush (again the T-bird on a FR 80 mast) would work as well.
You are looking at fairly wide boards, and boards that have a fairly wide range of use (windspeeds here) so getting a couple more sails (one larger and one smaller like using a 7.0 m2 as your basis then getting a 5.5 m2 and an 8.5 m2) would allow you to use the full potential of your board.
Hope this helps,