The standard lever is already very short. Have  you tried put bike in gear, roll back to get piston on compression and then kick..?

I bought a new lever for the rear tank models & it definitely makes starting the bike easier, doesn't sit in against the muffler properly but not by enough to foul your leg. It is slightly longer which does give more leverage

 

I have a 05 sherco 2.9. From the picture, your bike looks similar. It has a starting issue when it was warm as well. A buddy with the same bike had the same issue.

It can be frustrating!

I don't think the carb had anything to do with it as a sparkling clean carb with still be stubborn. 

I have a strong kick so that had nothing to do with it. I can tell by the sound during the first kick if I'm going to be there for 10 minutes letting it cool down. Gets a flooded sound. I've tried throttle wide open, partially, or none. No obvious pattern. Similar stuff from a friend...seems quirky. Perhaps the reeds? That's what going to switch next. Perhaps heat expanding cylinder tube?

Have you tried emulating a hot-start lever by introducing a vacuum leak? Motocross bikes have a cable-operated hot-start lever on their handlebars that operates a port on the intake or carb to bypass the carb and allow fresh air directly into the cylinder. On my 4 stroke bike, the carb had a vacuum port to operate the petcock. I capped that port when I installed a manual petcock. When the bike would flood too bad out in the woods, I removed the cap and it fired right up. Quickly replace the cap and ride on my merry way. I added a 12v solenoid valve to that port and wired it to an unused momentary button on my turn signal cluster so now I have an electronic hot-start button.

Found someone over on ADVrider with the same issue. https://advrider.com/f/threads/full-service-of-a-sherco-2-9-an-ongoing-thread.910355/page-2#post-22616841 I might be buying a 2.9 next week and doing a lot of reading on common problems and maintenance so I can make a pre-purchase checklist.

My Kick has got better. My riding has also got a bit less fearful going downhill, so I stall less. I open up the throttle more on the kick. It is no longer a problem three times a trip. It maybe is one time in three trips. If I move to where a rock is holding the bike up and I can put what energy I have into a kick it starts.

I blame my kick.

Try closing the fuel tap right after it stalled. It makes starting easier. Don't forget to open the tap after starting.