"But now if I start really quickly I pop a wheelie!!"
There's an old joke:
"Doctor, My arm hurts when I do this" (make a motion with his arm)..."can you help me?"
"Well, Mr.Smith...if you arm hurts when you do this" (makes the same motion as the patient, Mr Smith)...I suggest you don't do it..."
So don't start as quickly. Start a bit more smoothly by either feathering the clutch (if a manual) or a lighter foot on the hydro peddle.
If I'm starting up a hill (sans FEL), I'll lower the RPM to a fast idle to start, then move the hand throttle to PTO RPM smoothly as soon as I'm in motion. It'll take some practice. This is necessary since I do mow a rather steep embankment (Drive up and reverse down). I don't have any suitcase weights yet either. However, my 790 does have the grill guard and weight bracket installed.
I looked at the picture of your tractor (on the link you so thoughtfully provided). I reckon you'll have to call your dealer to see if a weight bracket is available for your tractor. Or, a local welding/fabrication shop can manufacture one for you (suggest you have at least one suitcase weight available for the fab shop). Be advised this bracket would be quite heavy in order to take the weght of the suitcase weights. It will be a bear to remove and reinstall (buy a cheap roll around engine hoist for this task).
However, the solution may end up being just a change (as described above) in your starting technique.
My cutter (60" unit), when raised, really does lighten the front end. To make matters worse, I have to drive up a mild slope (5°-10°) when I'm returning from the field. I tow the cutter (top link loose or disconnected...PTO off) when I go up this slope. I'll raise the 3PH to keep the forward end of the cutter from digging in and the tail wheel carries some of the weight. This method may help you as well.