Probability of getting long-duration multiple entry Schengen visa

Probably not too bad, even though it's all very speculative and there are no guarantees. Unfortunately, if you look at the statistics, the Greek consulate in Kuwait seem a little more stingy than other consulates in the same country.

The Netherlands, Germany, France, and even Austria seem much more generous. One strategy could therefore be to postpone your Greece holiday and visit Austria (assuming you are not interested in going to France or Germany again). However, do not cook up some fake itinerary just to get the visa if your real intent is to go to Greece. That's a dangerous strategy and could backfire badly, annihilating the great history of compliant travel you have been building in the last few years.

But you should not put too much weight on such statistics. In theory, visa decisions depend more on your circumstances than anything else and differences between consulates could also result from the type and quality of application they receive. Importantly, short of a compelling professional reason to visit the Schengen area regularly, there isn't much you can do to strengthen your case (beside having a reasonably strong application, which seems to be the case given your past successes in securing visas).