I ended up with a $100 Pixel 5 that I loaded IodeOS on and stripped everything out but phone, messages, Signal, settings, and contacts. I'm using Before launcher to make it as simple as possible.
The pleasant surprise is the battery has 1200 charge cycles on it, and I worried it would need replacement...but with so few processes running, I'm getting 7 days of battery life on a 6-year old phone.
It's still too big, but it was the smallest thing I could find that was guaranteed to 100% work with a Lineage fork.