The American ex-pilot on the top left had some experience with airshow flying. His theory about what could have happened is a partial G-LOC that happens when transitioning from negative to positive G.
This British YouTuber was also talking about the same thing. That negative to positive G transition and partial G-LOC. Which is just as deadly as a full G-LOC and have killed pilots before.
Based on these two channels, the theory is that when Wing Commander Namash Syal had suffered a partial G-LOC from negative to positive G transition, and made a catastrophic mistake at low altitude. It started when he was making that banked flyby for a photo op, where he was subjected to negative G. This pushes blood to his brain but also dilates the blood vessels in the body. Then when he turns for a barrel roll, he is suddenly subjected to positive G. This suddenly rushes blood from his head back to his body. Because of vessel dilation, blood moves too fast for his G-suit to counter, so he could have suffered a partial G-LOC during that moment. That's where he made his mistake and turned the aircraft nose too low during the barrel roll. But unfortunately he was already too low, and there wasn't enough altitude for him to recover, thus the crash.
Both channel say that airshows tend to be performed by younger pilots as their bodies can cope better with the G-forces. Namash Syal was in his mid-30s, so he should be fit enough. But Sergey Bogdan is flying Su-57 air shows in his 60s. So the age theory doesn't work so well.