Hello,
I handcycle with a Stricker Lipo. You have the right ideas.
1. Change the amount of pressure of your tire to increase resistance. Good for hills, bad for flats.
2. Change your center of gravity more forward to put more weight on your front tire, again increasing resistance. Sounds like you've done as much as you can so far. Another option is adding another axle further back.
3. Add weights to the front axle. Bad for amount of power required uphill, but you will be able to climb steeper hills (gearing/strength becoming your new limit instead of traction).
4. Reduce total weight of your rig. Since your handcycle probably cannot be changed....that means you
5. Get a power assisted unit. Very expensive, but honestly, I get a better cardio workout when I use the electric assist instead of all muscle.