The acronym "TOW" stands for "Time On the Water".
The more TOW you get on virtually any board, the better you will understand the ways
you need to interact with that board.
Foot positioning, overall weight positoning, how soon to move back to the straps, how to initiate a carving jibe.... pretty much all aspects of windsurfing technique need to be
to some degree "board specific". In general, yes the techniques are all pretty similar, but the more you can learn about ech of your boards the better they will serve you.
Getting a larger rig 8.5m2 to 9.0 m2 will get you alot more time planing on flatter water which can be very enjoyable indeed.
If you feel your 7.8 rig is "heavy", what is the carbon % in your mast. That's where you can save the most weight.
5.0 m2 will work on the Carve 99 but for true 5.0 conditions a board in the 85-95 liter range would be even better.
Yes, after you sail the new boards a few times, you will begin to understand that if you go out on the 99 liter with your 6.5 and aren't planing all the time, simply change boards to the 121 liter and you will be planing.
Conversely, if you go out on the 121, and you a a little out of control, the board is bouncing around alot, switch to the 99 liter and you will have a much better time.
You could use a handheld windmeter to help you decide which board, but you will soon learn to look at the water (and what other sailors are on) and will begin to select the right board for the condition.
I'm a little puzzled why you are tacking on your Carve 145. Jibing is so much easier and faster. The only time the fin can be "pushed by the waves" is when you have no speed.
Try to work on keeping your board moving as fast as possible all the time.
Tacking entails a near stop when directly upwind, but if you jibe, your board should be moving all the way around, and you won't loose much if any more ground to windward by jibing on your shortboard.
If you tack a shortboard, you will stop and drift downwind for at least a few moments,
if you make the same turn, only jibe, you will be around quickly with very little loss of speed and won't drift downwind as much.
As far as "transitioning" from the 145 to the 121 and 99, they all saill fairly much the same, but lateral (side to side) stability and weight positioning fore and aft will become more critical as you learn the smaller boards.
So, foot positioning will be a little different (stay more over the centerline, and move back on the board to the straps more slowly.)
It's an adjustment that takes a few sessions, but it will be well worth it when you go faster, and can enjoy higher winds and rougher conditions on the smaller more agile boards.
Hope this helps,