On the OPT side I have the white with blue stripe wires soldered together for later use.
The OPT's blue /white wires are for the ultralinear screen grid connections, they should
not be soldered together.
This would create all sorts of "weird" problems...
Separate them, and tape over, or use shrink tubing over the ends of these leads.
Your shielded twisted pair (STP) is good, with the drain soldered to the black wire.
A good convenient grounding point on the board would be either(feedback) J10 or J11, pin 2. Those connect directly to the board's signal ground.
Just connect either one to the chassis plate, that should do the trick. The board mounting screws on either side of V1 ( and very close to J10 / J11) would be a good, direct connection.