[@bitcoinintern] I'm fairly confident you're right that it's an exchange and I posit that it's Coinbase. I traced 30000 coins from the GDAX wallet in 2017 through until it started spending the 546 dust that the "how much you donate anonymously" spam tracer put into addresses. this directly led me to the 1a6 1.5 million coin address. I think Coinbase are consolidating UTXOs and read on X lately that they are eating a $20 million loss on BTC UTXOs that were never consolidated.

1Ly9NwZK8JRkzPyrDYAFunhcQ5asH42HGv

I got lazy after spending about 20 minutes per day clicking through each withdrawal. The 546 sat dust tracers short circuited the chain right back to the big address