Re: Essential fly fishing gear/accessories.
Since you asked for essentials, I'll try to stick it.
Rod and reel, check. Flies, check.
Tippet spools, 3, 4, 5X at least. You can get away with the majority of trout fishing with these, and rebuild a leader to some extent.
A spare leader, 7' 4X. Can be easily easily extended to 9' 5X or cut back for heavy flies.
That might be barest bones list of essentials. But there are a few that I consider essential.
Felt wading boots
Further down the list are things that make a day more comfortable.
Cold beer waiting in the rig.