The bouncing voltage at stock is normal.
The ~1.5V you are seeing at idle must be when a single/low thread count load occurs, CPU may be boosting to ~4.35GHz, hence uses higher voltage.
Under IBT you see ~1.25V as all / high thread count load occurs, CPU will not be boosting to ~4.35GHz, so uses lower voltage.
CPU temperature on Ryzen is not an averaged result of all dies sensors. There are 20 on die sensors, some rotation goes on which is displayed plus highest value is given precedent to be shown. So some bounce, odd behaviour compared to say other systems is expected.
When Ryzen is pushed +25MHz over base clock, in your case 3.7GHz it will enter OC mode, VCORE will go to ceiling voltage of PState 0.
I would change only PState 0, leave CPU Core Voltage on defaults on Extreme Tweaker page, adjust VID in PState 0, make sure Global C State control is set to [Enabled] in ZEN Common options menu, you should see VCORE/MHz downclocking even with OC'd PState 0.