It just dawned on me to look at the video output. It appears the load is 15K ohms. This makes the charging time constant with C403 1.5 ms, but it has to be much smaller to charge on the sync tips (about 300 times smaller!). Therefore, leave R409 as 470k, but reduce the size of C403. It may not be practical to reduce it enough to fix the charging time constant without affecting the video response due to the high value of R406 - you will need to experiment.
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