On your Carve 145, getting into the harness, and the footstraps, kinda go together.
The board really won't plane well until you are back in the footstraps (at least having your front foot in the front strap, for sure) and you really can't hook in and sail efficiently further forward on the board.
Here's a couple of suggestions.
First, since you are extremely tall, what prevents you from rigging your sails up higher on the mast base extension? If you already have the boom as high as it will go in the boom cutout in the sail, you can "check out" what a higher boom would feel like by simply adding some extension under your sail.
Yes, I know this is not optimum, and won't allow you to "close the gap", but at your skill level that may not be as important as getting you comfortable in the harness.
Harness line length is not so much a function of your height or tallness, as it is a fucntion of your arm length, the type of harness you are using, and to some degree the type of sails and the sailing discipline you are doing.
So, if you try the higher on the extension sailing and it works for you, then you can take your sails to a sail loft and have the luff sleeve cutout moved a little higher (as long as it does not interfere with any battens or cams.
Try to move back slowly and progressively on your board until you can get your front foot into the front footstrap, and your rear foot on the fore and aft centerline of your board between the front and rear footstraps. Since you are so tall, you may end up with your rear foot very close behind the front foot as you have to place that rear foot at a position that places your weight correctly to "trim" the AOA of your board so it will take off on a plane.
Keep your weight a little too far forward and the nose won't lift enough for the board to plane off freely (less than optimum AOA here).
If you place that rear foot too far back, the nose will be too high and the board will be pushing too much water to plane off efficiently (too much AOA in this case).
Just try to move back at a rate that matches the acceleration of your board, keeping the board very flat (rail to rail) and sheeting your sail in and raking it back in a way that keeps your board accelerating.
When you are ready to plane, transfer your weight off your rear foot and onto the rig (through the harness lines by hooking in) so that you do not put any weight on your front foot.
The board should slide onto a plane pretty effortlessly.
Hope this helps,