Please tell me, could i use an emergency doctor appointment for thrush?

Thrush: This is a common, non-emergent condition. You can be seen within a few days of onset and be treated with an anti fungal rinse or anti fungal tablet rx.

At your own risk: Insurers are now looking at visits and refusing to pay ER level visits when the issue is simple & not an emergency. This could stick you with a bill for thousands. This would qualify as a irritating problem but not an emergency. If you went, they would triage you to the bottom of the "need to be seen" pile and expect to wait a few hours.