I'm not sure I fully follow, so apologies if this is tackled in your post: Finding the boar is what triggers the bite night. If you avoid it you should be able to postpone the reveal for quite a while. (Going to the Underdark or the Mountain Pass will also trigger his confession about his vampiric nature, so avoid those too)
I was actually able to blindly experience the Gandrel scene not knowing that Astarion was a vampire during my first Early Access run, because of the way I did quests.
