[ngoma]

Minimum allowable T-stat opening is 8mm (fully opened).

Did you follow the steps to purge entrapped air when you refilled?

In the Volvo, that entails disconnecting the small upper hose from the the cold start waxstat, holding it level with the expansion tank, and verifying no more air bubbles are exiting.

As an experiment, you should be able to do this with the engine running, and by revving up the engine, see increased flow correlating to the engine speed. Have a helper refilling the expansion tank so it doesn't run dry. This would tell us that you have at least some functionality from the WP.

Normally the lower portion of the radiator will be cooler, but it shouldn't be cold, especially when the engine has been operating for some time under load. Did yours stay cold even after driving it (more than 10 minutes)?

Did you verify the gauge indications with an IR thermometer? You can aim it at the engine block next to the temp. sensor, at the radiator hoses, at the radiator tanks (upper and lower sections), at the radiator hose flanges, and take notes.

REMEMBER, IDI diesel engines do not come up to operating temperature as easily as gassers, when started from cold and simply idled. They require a load.