I just wanted to ask whether or not this is a good circuit for an MCU controlled triac dimmer. I am very unsure about the optimum values for R2 and R3. Right now they are very high, so I think you could actually short the load pins without problems. I consider adding a thermal fuse as well, but where would I place it, and what value should it have? Are there any other obvious issues here or something that I should improve upon? Mains here are 230 AC 60 Hz.
Your triac is across the AC mains, it will explode with considerable violence and take several fuses with it.
On almost all circuits I've seen on triac dimmers, they have been wired like this (like here: https://www.instructables.com/id/Arduino-controlled-light-dimmer-The-circuit/). And I've done several spice simulations. I must have seriously misunderstood then.
The circuit in that post looks good at a first glance.It has little resemblance to your circuit. Do note the second optocoupler for a start (for the zero crossing) and how the triac is wired very differently.
That's because you have a 300k resistor where your load should go. A halfway decent triac can do a few amps at least.