What to do with 100% Exeggcute from quest?

I would evolve it to see what moveset it gets and keep it as a trophy. I've been sitting on a 98% ex/Psy and a 96% ex/solar K.Eggy from raiding; but I can't bring myself to invest the dust when I already have enough powered upeggys and can catch weather boosted exeggcute up to L35 and Evo with no dust investment.

I've been sitting on a 100% Exeggcute from quests in case it ever becomes possible to evolve it into the Alolan form. Like you, I don't need more powered up Eggys at the moment.

You don't need it. So keep it as a collection pokemon. If it becomes useful in the future there you have it.