This is more of a subjective thing. Up to interpretation. But I do have some advice.
First, don’t use una corda JUST because a part is soft in a piece.
Second, don’t be so confident in your dynamic abilities that it becomes a virtue to not use it.
Use it when it feels right. Heck, you don’t need to use it at all.
Or you could use it a whole lot. Experiment and see if you like it.
Your left foot should be on the una corda the whole time, even if you never use in a piece. Many people use their left foot for support by putting it on the floor, but if sitting correctly you will have all the support you need. Moving your foot off the una corda pedal for support on strong chords should be an exception, not a rule.
Una corda does not just make things sound quieter. It creates a whole different color in the sound. This is partly due to the fact that where the felt hammers strike the strings will be more compact, and the una corda area less.
Finally, the una corda, damper, and sostenuto pedals are NOT a on/off switch. They are a dimmer switch with infinite possibilities. Too many people forget this last part.
Topic: Clair de Lune Pedalling