That climb request to FL380 "to avoid weather" has been puzzling me.
(begin speculation)
Now I think the pilots were not overly concerned with the weather, before flying into a developing cell.
Consider, at the time, there were up to seven other planes nearby, including several flying just ahead and just behind QZ8501 on the same airway. According to media, none of the other pilots reported flying through strong turbulence or any other hazardous conditions.
It seems to me that QZ8501 was a routine flight. There was some weather ahead so they requested deviation to the left, like they've done hundreds of times before. From the radar picture you can see Emirates 409 had done the same just ahead. In fact, QZ8501 was joining the track UAE409 had left a few minutes before.
Then they requested climb to FL380 not due to "weather" per se, but because it was routine at this stage of the flight to climb higher (being lighter with less fuel). I've reviewed several other A320 flights on this route and it's not unusual to see flights go to FL360 or FL380 near this spot.
The routine aspect of this climb is what makes me think the crew was not overly concerned about the weather or the situation they were in. If they were seriously concerned, they would have deviated further first, not climb up at that point.
(I can see pilots climbing to better assess what's ahead, and maybe for a smoother ride, but I highly doubt a very experienced Captain from the tropics would try to out-climb a cumulonimbus.)
Yet the radar picture shows QZ8501 at FL363 and climbing (*). Perhaps control was already lost at this point. By deviating left, they inadvertently flew into a newly developing CB with very powerful updrafts. The plane is violently pushed up, nose very high, and quickly rolled to one side; groundspeed rapidly drops.
The pilots tried to recover from the unusual attitude and were not able to do so.
The plane did not "break up in flight", but flight control was lost. Perhaps the rudder or other control surfaces sheared. As the plane was plunging to the ground, below FL290, the ADS-B signal was lost due to line-of-sight distance from the nearest ground station at Pangkalan Bun.
The aircraft was largely intact when it impacted the ocean, perhaps steeply nose-down, creating few debris. The pilots tried to save the airplane all the way down.
(*) also note that ATC FLs are not actually physical altitudes but derived from Mode C pressure transmissions, which may be erroneous.
(end speculation)