What I will say is that His audience took him literally, and expected His return before the passing of the present generation.I am not competent to interpret what Jesus meant, and I won't speculate on it. What I will say is that His audience took him literally, and expected His return before the passing of the present generation. The first letter of the NT to be written, First Thessalonians, attests to this. Paul was dealing with a perceived problem in Thessalonika -- why were so may brethren dying physically (had they messed up in God's eyes somehow?), and would they be disadvantaged compared to the rest who would be alive at the Second Coming? It's tough to explain this in any other way than the literal interpretation of this oral tradition on Jesus's pre-Transfiguration words, later recorded in the synoptic passages you cited.
[Mat 4:17 KJV] 17 From that time Jesus began to preach, and to say, Repent: for the kingdom of heaven is at hand.
Jesus did say to expect the kingdom but that does not resolve why those standing there would not see it