Humidity drops from 90 to 40 percent by midday even with a full drainage layer Lighting & Humidity

Cover the screen top. That is the whole problem and everything else is secondary.

A full screen lid with a heat lamp above it is a chimney. Warm moist air rises straight out and dry room air replaces it, which is exactly the curve you are describing. The drainage layer cannot compete with that, because a reservoir under three inches of substrate has almost no evaporative surface exposed to the air. It exists to stop the substrate going anaerobic, not to humidify the tank.

Cut a piece of glass, acrylic or even heavy duty foil to cover 60 to 80 percent of the screen, leaving a gap along the front and around the lamp. That alone usually takes a tank from your numbers to something like 70 to 80 percent through the day.

After that, if it still swings, deeper substrate and more live plants both buffer it. Plants transpiring is a genuinely significant humidity source in a planted tank.

Also check what you are measuring with and where. Cheap analog dial hygrometers are frequently off by 15 or 20 points and they respond slowly, so a reading of 40 might be a reading of 60 taken badly.

Use a digital probe, put the sensor at animal height rather than at the top of the tank, and if you can, calibrate it once with the salt test. Half the humidity panics I see turn out to be instrument problems.

The heat lamp is doing a lot of the work here too. If your animal does not strictly need a basking bulb, switching to a lower wattage or to a different heat source will flatten the curve considerably. A big daytime swing from a bright hot bulb over a screen is fighting exactly what you are trying to achieve.

For what it is worth, a swing is not automatically wrong. Most species do better with high humidity at night dropping through the day than with a flat 90 percent, which encourages exactly the fungal issues everyone else worries about. 90 in the morning down to 60 by evening is a healthy pattern. Down to 40 is too far, so you do want to fix it, just do not aim for a flat line.

If you go the automation route eventually, a small ultrasonic fogger on a humidity controlled outlet is cheap and works well. I would still fix the lid first, because a fogger fighting an open screen just makes your room damp and your electricity bill higher.