That warmth made me wonder if it is okay to use the Qi charger as my primary charger, and to leave the phone charging indefinitely.
Yes, you can do this.
It is perfectly safe to leave your phone charging on the wireless charger. The warmth you feel is perfectly normal.
From the battery perspective, iOS will manage the charging of the battery with the Battery Health feature.
All iPhone models include fundamental performance management to ensure that the battery and overall system operates as designed and internal components are protected. This includes behavior in hot or cold temperatures, as well as internal voltage management. This type of performance management is required for safety and expected function, and cannot be turned off.
How Qi Chargers work
Qi chargers work through a electromagnetic property called induction whereby a magnetic field passed over an electrical conductor produces an electromotive force. In other words, a magnetic field passed over some wire will produce voltage and vice versa. These chargers work by creating a electromagnetic field by passing voltage through a coil (transmitter). Then that field is put near another coil to produce voltage (receiver).
Inside your phone is a coil of wire, that when lined up with the charging pad/mat that has it's own coil, will produce 5V of electricity to charge your phone. This coil is connected to the charging circuitry of your phone along with the USB charger connection. That voltage that's created by the coil is sent by the charging circuit to your battery where it charges until it's full; then it stops charging as it would if you had hooked up your device to a USB charger. I go into this into more detail in the answer to Are iPhone Wireless Chargers Good for Batteries?
This is no different than your USB charger remaining plugged in when the device is fully charged.
This topic has been asked and answered here on Ask Different already. Whether the current is coming from a USB/Lightning port or from a Qi charging coil, the battery doesn't know or care; the electrical properties are still the same:
Now, the warmth that you're feeling is normal. The coil is still producing electromotive force if the charging pad is not turned off. However, that voltage isn't being used so it has to be dissipated and it does this via heat. Additionally, the simple act of moving electrons down a conductor will generate heat through friction. This is perfectly normal. In fact, on regular mains sourced power supplies, those too become warm as the transformer (what goes from 120/240 mains to 5VDC) continues to operate with nothing connected.