There really aren't that many modem/router combos out there you can actually flash with open source firmware.
Router? Sure, no problem.
Modem? Outside of some very niche Fritzbox devices and others listed on OpenWRT Table of Hardware
There really just aren't that many.
So, fine. I will use a proprietary modem.
I will put it into bridge mode, let it decode the DSL analog signals into ethernet and feed that into a router of my choice.
Question is: Which modem?
I don't want two devices heating up my shelf. I just want a tiny modem that converts analog to digital into an RJ45 and then let's the router do its thing.
Does such a magical device exist, because my search on this front has proved largely fruitless
Is there any reason you are not just calling your ISP and asking them to put their box into bridge mode and then setting your own router up behind it?
You control everything from the router and into your local network.
If you can't trust your ISPs "modem" in bridge mode, then you also can't trust the infrastructure beyond the modem, and in that case you would need to set up a privacy respecting VPN service directly on your router to encrypt the traffic your ISP sees.