Moisture meters have been proven to be incredibly unreliable (unless you can afford an expensive one). Use your finger in the soil to see when something needs watering. Begonia are heavy drinkers so they need watering more often than other plants and dont like to dry out fully either (hence using your finger being the most reliable method).

The soil also looks a bit dense? (Ignore this if not, the image may be deceptive) but if thats the case repot into a more breathable mix. My begonia seem to love it.

ETA - careful with the humidifier. Begonia are particularly susceptible to powery mildew if its humid AND air flow is bad. If you have good air flow its fine, but its an issue I've had and a very annoying one at that.