I did a lot of lighting retrofits at the hotels before I touched any IoT, so this one I know.
Get a constant current buck module with a dim input. The cheap ones built around a PT4115 cost a couple of dollars, come in 700 mA versions (or set by a sense resistor), take 12 to 24 V and have a DIM pin. A Mean Well LDD-700H is the nicer branded one. Feed it 12 V, LED on the output, and your ESP8266 PWM pin straight into DIM. 3.3 V PWM is enough for both of those. Around 1 kHz PWM, no flicker you can see, the driver keeps the current flat as the LED heats up.
On the Hermetry side: add a command like light.set with a number parameter to the profile’s catalog as a Basic command, then a Slider control card that runs it. The slider sends once when you let go, not while dragging. For a light I switch off Ask before running, otherwise the shift lead gets a confirm box every time. Bind the current value to what the ESP reports back (I send light.level after each change), so the slider shows what the light is really doing. Control cards walks through it.