Hi Razz
You sound like me when I first started, and all the guys on here were feeding me tips. That was about 2 years ago.
How much would it improve my pointing to get into the straps.
Yes, getting into the straps (and applying the right forces) will get you blasting up wind. That's how the board is designed to work.
If I aadjust my lines to 1/3 down the boom I generally get a lot of load onto my forearm.
You should have even pressure on both arms, and be able to commit all your weight in the harness. If not, then either your sail isn't rigged right, or your harness lines aren't set up. I own a Koyote too. They are a really nice easy to rig sail.
Is the idea to lean out sideways and try to keep the mast straight up?
Not true. I was doing this too when I was first learning, resulting in catapults and loss of power.
..because if I lean back to much I end up going of the plane and stalling
Yes. For a freeride, straps out board, racking back the sail when in the straps is how you blast up wind, but there is a knack. When in the straps, you have to apply the certain forces using your harness, arms, leg/foot etc. This isn't intuitive. I had to be told how to do it. What you eventually want to do is sit in your harness and pull the centre of effort of the sail back and use your rear foot to push the board on the opposite rail (aka trimming it). There are forces going through your harness and through your back leg. Very little force through the front leg and arms. This is very hard to explain in words.
I measured my lines and they are 80cm overall with a one hand gap inbetween.
They are big harness lines. Are you trying to get into the straps by making your harness lines longer ?
As a remedy I either move the lines 5cm/2in further back or should i try to find longer lines
I reckon just give it time. As a few guys here told me: Concentrate on your front straps first. Don't worry about pointing up wind or trimming the board. Just get used to riding in the front straps. The front straps are the easiest to get into.
If you do a search for my name, you'll find some of my newbie questions. I was asking very similar questions. Good luck

J