Don't be so sure. Einstein, in spite of all his achievements, was human. He made mistakes and misunderstood some things. Overall he understood physics more deeply that any of his time, but from his point of view maybe it came so naturally that he never felt he had totally "earned" his recognition. That said, he was known for humility, but that doesn't mean it was disingenuous.

Isn't that the point of "impostor syndrome"? No matter where you stand, your measurement of your ability is subjective to whatever standards you personally hold, which may be different from an objective assessment? And maybe there is no true objectivity? Maybe it's all... relative?

Einstein already made his most significant contributions by age 26 and spent the rest of his life arguing against quantum theory, the dominant school of the 20th century. Most modern physicists regard the latter portion of his life a waste in way of contributions.

That’s a little misleading — the latter period started around age 40. E.g. Bose-Einstein statistics was from 1918, which would be age 39 if 1905 was 26. The EPR paper was many years later still. Also, you can’t leave general relativity out of his most significant work.