What's wrong with the combination of components in the title?

The issue I have may not actually be a problem but I'm curious to get another opinion nevertheless.

I had to find a circuit to connect an electrical sauna in the basement. The 1840 Watt sauna required a dedicated 120 Volt, 20 Amp, single phase circuit.

To do this I disconnected three baseboard heaters that were no longer needed in the basement. This gave me a dedicated circuit on a #12 wire. So far so good. However, because the original circuit was dual phase, I had to relocate the connection in the panel to a single phase 15 Amp breaker. Unfortunately I couldn't move the original 20 Amp breaker.

Now here's my question.

Since I have a dedicated circuit on a #12 wire, is it OK to use the 15 Amp breaker, especially if it doesn't trip when the sauna is in use? In other words, am I not "overprotecting" the circuit?

Looking forward to hearing your opinions.