Validate Triac circuit
It's probably safer to have D8 drive a small MOSFET to control the photodiode instead of relying on the current-sourcing capability of the GPIO pin itself. You should also provide a bit more than the absolute minimum 5mA cited by the datasheet.
A fuse should always be in the line - never only in the neutral. (Fusing both is OK.) If you fuse only the neutral, you still have a path from line to earth since in most jurisdictions, neutral is earthed somewhere. Dangerous and potentially lethal.
Your heater load is most likely inductive in nature, so you should consider the resistor-capacitor-resistor gate scheme shown on page 6 of the datasheet to desensitize the gate. You can always no-populate the capacitor later if you don't need it.
The device (without heatsink) has a junction-to-ambient resistance of 60K / W. Since your 1000W heater would draw around 4.34A of current when the triac is conducting, at 230VAC that's ~7W - at 100VAC it's more like 16.5A. You'll definitely need a heat sink :)