Well, I prefer riding when it is warm, or hot. When it rains it is still comfortable. In October when the rain comes, it is a cold rain. I hate cold rain, and temps at night can be in the upper 30s and lower 40s. I had nights like that on my tour across Northern Indiana and Ohio in June of 2019. It would get into the 40s at night. Not bad, but you have to be prepared. I don't see the mid to upper 80s to be particularly hot. I grew up in Iowa where it would get into the 90s and 100s and be humid as well. I ran a track meet when it was 106F. I moved to Pittsburgh in 1985, and that summer I kept waiting for summer to hit. It was in the 80s and I was waiting for it to get warmer to use the pool. I asked when summer hit, and they said this, "This is it."
The GAP and the C&O is heavily wooded. There is plenty of shade if you need it, and a lot of the trail itself is in the shade. There will always be bugs, especially on the C&O because, well, water.