I find it unlikely my ISP is breaking my quantum encrypted VPN just for ads

They don’t need to break encryption. You have an account to post here, and I’m assuming you are using the same device to access other services you have which also use accounts, right? Now, suppose your IP address doesn’t rotate very often, it would be very easy for a company like Google to relate at least a few accounts to start building a profile.

Then, since posting here is pretty easy to scrape, even more data can be collected… like the fact that you use a linux router and OS. The profile keeps getting bigger with every new interaction, and at some point, it won’t even matter if your IP changes, because there may be enough static data somewhere that could be linked to a different account (i.e. your phone, phone’s OS, screen size, etc. can be used to isolate you from other users).