After using mine for a few days, I found that the fan was still too loud when plugged in, even on the left-most Better battery slider setting, which sets the PL1 to a pretty high 15W. I also looked in the power plan settings but only found the Balanced profile.
To work around this, I enabled the Maximum processor state setting in the Advanced power settings menu (this guide shows how). I then created a new power plan named Silence and set Maximum processor state to 50%. This limits the CPU to about 6W & 50C regardless of the power slider setting, minimizing fan noise while still having usable performance. I can then quickly switch between this and the default Balanced power plan using PowerPlanSwitcher or as described here.
A first-party “Silence mode” would definitely be nice, but I’m happy with my workaround for now.