Hey there. I felt the need to chime in here, as I came along a similar learning curve as you seem to be going on now. If you recall, I responded to one of your posts on rec.windsurfing as well, a few months ago.
I started shortboarding on an F-Type 158 in late 2005, and switched to the 148 mid-2006. Roger and the rest of the F-Type fans on the forum were a huge help.
I concur with Roger that you need to get in the outer straps asap, on your F-Type. I had trouble with the outer straps at first, but took a 4-day course with Tinho Dornellas in FL in the US in the spring of 2006, and learned to sail in the straps comfortably on Fanatic Shark 160 w/ 9.5 and Shark 145 with 8.5 and 7.5. I came back to the F-Type and the first session was still tough in the outer straps. I moved to the middle the next session, got comfortable with that, and then tried the outer straps the next session, and never looked back. In the outer straps is when that board "comes alive".
With regards to your next board, be careful not to go too small. Take it from someone who learned on an F-Type and had some challenges going smaller, go too small too fast and you will not be happy. I would say do not go down the path of the used Carve. I would hold out for the Futura, or alternatively the new GO which has the exact same shape as the Futura, and is much less expensive. The Futura/GO have a much flatter rocker and will drive off the fin more than the Carves (I have not sailed a Futura, but did own a 2006 iSonic 122 that I liked a lot, and iSonic DNA is in the Futura/GO). The Carves had a lot more rocker (I owned a 2006 Carve 122 - before the iSonic 122 - and did not care for it, in the context of the learning curve from F-Type), and have much more of a "small shortboard" feel than I would get the flatter rocker and wider Futura/GOs would have. The Carve would be even more extreme than the negative experience you describe on the Sharks (which BTW the smallest Shark is a 130L at 70 cm wide, a Carve 122 is 67 or 68 wide if I recall).
My recommendation, with all due respect to Roger, is to get a 2008 Go 133 for use with 6.0/7.5, or at smallest a 2008 GO 122. Or get the Futuras in the same sizes if you can afford it.
And get on those outside straps on the F-Type as soon as possible.
For what it is worth my skills have progressed a decent amount, I now have a pretty diverse quiver, a Starboard Formula Experience 160 that I am upgrading to a 2008 Apollo Formula over the winter, 85 and 76 cm wide Slalom boards of another brand, and 65 and 61 cm wide freeride boards of another brand, sails 9.8 down to 5.0. (Had the Futura been out at the time I upgraded, several of those might have been Futura's, maybe new iSonics in the larger sizes.)
Anyway, good luck with your progress and quiver expansion. I offer my input as your path seems to be similar to what I went through over the past few years.