Lets start with this. The inability to hold pressure anywhere in the system will allow the temperature to rise once vapor begins to form in the system.
See if you can get your hands on a tester. They usually have adapters to check caps as well as the system.
Looks like this
View attachment 1716113661
Side note: A radiator without a recovery tank will be full cold about an inch below the cap.
With Regards to timing:
A. Its seems close enough that should not be the cause of overheating.
B. RPM RPM RPM
The RPM of the measurement is important. Centrifical advance adds to the intial above 600 or 650 rpm.
So 2.5* BTDC at fast idle is retarded.
2.5*BTDC should be at slow idle specification. I'll guess around 600 rpm is the spec. Could even be 550 rpm "with lights on" (so the alternator is adding load).
No vacuum advance at idle is correct.