I’d assume 8m would be your favorite boosting kite then, or if you want more lowend then 9-10m. I’d pick a nice structured delta like XR, Pivot or classic Rally if you can find one for boosting/TT kite.
Foiling can utilize same quiver and I’ve been of Rallys as a “single model covers it all” but UFOs are just too great to foil with. At whitecaps (16 knots) and slightly below you should be able to foil on 5m, under 10 knots wants a 9 but you want proper line.
So 7/9 UFOs (or 7/10 Kaupers) plus 8 or 9 Pivot/XR for boosting.