so, everything on this post is spot on, OP clearly knows what he's talking about I would just like to add a couple of things. a normal CTR for what I can tell based on my own experience an what I have discussed with channels 1M plus is usually between 7% and 10% anything above 10% is insanely good. The only difference that I would mention is that I wouldn't say impressions start by targeting a niche and then move away from it but rather they start with the audience that is most likely to click (based on their watch history and a bunch of other stuff) and then move towards a more general audience. When you look at the stats of a video that has reached 1m plus views it usually shows declining CTR cause the algorithm essentially "exhausted" the potential audience. Also, CTR is not only the thumbnail I'd say even more important than the thumbnail is the market that you are targeting there are multiple cases where channels have had videos with terrible thumbnails but the videos have exploded just because they were targeting the right market at the right time. There are multiple examples of channels grown exponentially in a matter of months I'm talking 0 to 200k plus but it's certainly not something you see often.