Project car: assuming bad starter relay. Confirm?

I do get voltage at the “Sol” post when I connect a jumper wire from yellow Ignition post to “sol” and only when I twist key to “start.”
That may not be large enough wires to carrythe current needed by the starter solenoid.

Jump it this way - which more directly feeding the starter solenoid from the battery.
That's what the erlay should be doing internally.
upload_2020-3-26_12-4-11.png

Of course make sure the car is out of gear and hand brake on.
A screwdriver with insulated handle will work.

If the solenoid doesn't engage or at least click, check the starter and rechack the battery and battery cable connection.
A charged battery with no load should be 12.5 to 12.8 V
During starting it may drop. if it drops below 9 or 10 its on the weak side.