creepage clearance
As you posted the question in the "Professional Hardware" forum, I assume that a PCB coating is intended anyway. This would solve the issue.
Furthermore, according to IEC 1010 creepance distance rules, 0.7 mm would be O.K. if at leat the board is coated by solder resist.
Because the triac is self-triggered in case of overvoltages, no overvoltage category must be considered between triac
terminals. You also mentioned "to-heatsink" creepance distance. Depending on the heatsink potential, different requirements
may apply here, e.g. reinforced insulation.