The problem with this idea is that the situation you propose (Lithuanians uniting the Baltic tribes behind fighting the Crusaders and beating back several crusade waves, as well as forming a state which is seen as being able to stand up to the Christian world) happened almost word-for-word in OTL, and the Lithuanians converted anyway. The reasoning is simple - being a renegade religion-wise
sucks. Adopting Christianity allowed Lithuania to establish peaceful contacts with the world around it, more easily take in Western culture and technological advancements and, most importantly, stop the brutal, hundred years long war with the Teutonic Order. And that war was
brutal. III tome of "History of Lithuania" calculates that by 1400, there were almost as many Lithuanian and Lithuanian-born slaves in Teutonic territory as there were Lithuanians in Lithuania itself (200k former and 300k latter), that's just how painful the conflict was.
So if the target is to have paganism in Europe survive as long as possible, the goal should be to not strengthen Lithuania, but to weaken Christian Europe, so Lithuanians or any other pagans still left had less of an impediment to convert. I've already posited in
this thread that a Europe which was thoroughly thrashed by the Mongols could potentially see Lithuania remain pagan for a much longer time, as the Teutonic and Livonian Orders would be gone and Orthodox pressure might not be enough to have the Grand Dukes convert to it, especially if Lithuania ends up expanding both to Catholic and Orthodox territory.