GM:
As for crafting thermal clips with omni-gel, if that was possible wouldn't that have been in ME2 or ME3? Instead you had to find more or perform melee attacks.
True. Which means any new developments depend on the state of the ship's fabricators and tech lab - which in turn depend on how much you want ME tech to spill out onto Thedas.
GM:
Thermal clips were not invented in relation to the Reapers. The geth created them to eliminate inefficiency. It's faster to eject and swap than to wait for a cooldown.
I meant in time, not in reasoning. Before Sovereign made its move and deployed geth forces using the new tech, which spread across the galaxy as soon as someone got a close look at geth weaponry.