My personal experience with quite a few Ontario fly-ins.
If you want walleyes, go as early as possible. You’ll catch hundreds and hundreds. Most times, the biggest fish are 5-7# but bigger ones too.
Late spring into summer the fishing is still great, but not all concentrated at the nearest incoming River.
Midsummer, I’m not too impressed. Yes, the fish are there but I hate fishing in the heat.
Fall can be tougher to find fish but once you do, it’s like Spring again. I’d rather hunt though.
I don’t fly in for smallies, but they are everywhere there are walleyes, plus many other places too.
I did a late fall fly in for pike one time (not Ontario). Weather was horrible, but lord we hammered big fish.
For BIG walleyes or Muskies, don’t fly in. (Except Lac Seul).
Bucket list. I’d like to catch huge Lakers and giant pike at one of those fly ins in the NWT.