App store rankings! I've seen some posts about this on reddit, and there is some bot on telegram that tracks it but only for Coinbase. It is such an easy metric. If you can find data on appstore rankings for previous cycles you will see that this is one of the best if not the best indicator by far.
Yeah, it's already mentioned in the posts above.
It's not necessarily what I was asking in the OP, but historically it was a good metric to monitor the market getting overheated and to decide when the peak of the cycle was. But I don't think this metric will work forever. First of all, the old retail investors likely already have crypto apps installed. Second, Coinbase is no longer as much of a hegemon as it used to be. Looking at the data:
https://apps.apple.com/us/charts/iphone/finance-apps/6015?chart=top-free
we can see that Crypto.com or Kraken are not far behind, meaning the popularity can be more split over a few crypto apps, unlike in the past when it was concentrated in Coinbase.
Thirdly, other non-crypto financial apps now also offer crypto investing, so people could just stick to them for convenience.
Your interpretation is wrong. Almost nobody knows what Bitcoin is or how it works. Most people who are in "crypto" don't know it either. 
Technically that's true, but let's not pretend the average retail investor actually cares about anything more than buying/selling BTC for fiat.