[Penthode]

Here is how I would revise your circuit. The the upper resistor should eliminate the sync clipping and the 680K diode bypass resistor will limit the clamping and can be varied to the degree of clamping required.

It occurred to me afterward that because the CRT is conducting, the diode remains forward biased even with no sync or video. Nevertheless the bypass resistor should make the brightness level less likely drift over time.

I guess all of us are suffering cabin fever!