The diode only will serve to make a voltage offset; will not isolate anything (unless the focus grid supply current...). Is better to invest in the resistor values.

C524 is probably stacked on the B+ instead of going to GND like
most sets so 50 V rating used. Fisher & GE also did this. Most
others used a 4.7 or 10mfd 315 V to GND.
Zeno

Okay, I thought the current loading was the issue, but I guess you're saying that it's actually a voltage drop on the screen?

The issue I'm having with this is that when I put the 1M pot in the circuit with the other side to ground, it creates a load on the screen supply and the screen dims. I thought it was a current issue, but I guess it would make sense that it's creating an additional resistive divider for the screen potentiometer. There is a lot going on in this circuit, and I'm having trouble visualizing exactly what happens when I put the pot in. It's like this, and the arrow is where I would put the proposed diode in place:

https://i.imgur.com/iyBW4v7.png

Otherwise, are you saying that I can prevent the voltage drop by using a higher value resistor to supply the Focus Grid, with another one to ground on the other side? I think I mainly need help understanding exactly why the G2 is becoming dim in order to fix it because I don't know how to do the calculations for the resistors if that is so.

Yes, it's stacked on the B+. There's a .01uf film or ceramic (I'm not inside right now) going to ground between B+ and 162V Source. I guess I could try replacing that instead, or increasing it a bit if it looks okay. I've seen some film caps go bad I guess, but it's very rare.

If focus adjustment are great at near top, you can add a some Mohms resistor from "cold" wiper from potentiometer to ground (in series with potentiometer), reducing the loading, and making the focus adjusment less coarse.

The actual voltage at the 530V boost line measures at about 560VDC at the source. The focus looks best at something like 400VDC.

Good, so is possible to add the resistor.
BTW: I wrote "in the wiper" (perhaps making some confusion), so I can explain better: the resistor needs to be added in series with the ground side of it; the pot. pin dedicated to low voltage, and potentiometer being only a part of the divider. Like a 3M3 resistor in the ground side, then to the one leg of pot. The movable part of it goes to focus grid, and remaining part of the potentiometer simply goes to 530V. Total, more than 4M, better than the only 1M from potmeter.