Hi again jsw225,
At your weight, you need a larger rig, and you need for the rig to support as much of your weight as possible.
If other sailors are on 7.5 m2, you need at least 8.5 m2.
If you "sit down" on the harness, the rig really cannot support your weight and your weight is going down onto the board.
Why? Because the rig is pulling mostly downwind (with a small forward force component)
away from you.
So, if you lean back away from the rig, perhaps "hook" the top of the rig upwind a little
(basically this is not normally a good thing, but at your weight it's probably going to be a requirement until you get fully back in the straps and are sailing rigs large enough to fully support your weight) the rig will be able to support more of your weight.
So, if the rig is mostly pulling away from you, toward downwind, and across the board, "sitting down" just pulls down on the boom (alot) and this does not support your weight, and keeps lots of weight on the back of the board.
If you lean back, away from the direction the rig is pulling, hook the rig upwind a bit, the
forces generated by the rig will be in direct opposition to the forces you are generating with your upper body weight. This will result in the rig "holding up/supporting" as much of your weight as possible.
"Sitting down" when you aren't in the footstraps, can be a good strategy to avoid catapaults, but it will not result in good form and good speed as you move back on the board.
As Adrien suggests, as your board speed increases, you can move back progressively on the board, eventually getting both your feet in the footstraps, with a "figure 7" stance (body leaning back with your arms nearly straight out from your shoulders and fully extended), and steering your board completely with your feet.
You are a ways from this goal, but it sounds like you are getting there about as fast as anyone can.
Right now, your weight is a bit of a handicap, but once you figure out how to move back on the board progressively as speed develops, what size rig you need to support your weight, and how to get smoothly and progressively into the footstraps, your weight will become an advantage as you can resist significantly greater forces (rig pull) without getting flung/catapaulted forward. When you get to this point, "sitting down" will no longer make any sense.
Hope this helps,