dist-upgrade shouldn't try and move you to another release version, do-release-upgrade alone does that, as far as I know. Things get a little more confusing using apt update/upgrade vs. apt-get update/dist-upgrade, they don't seem to be quite the same in all cases for me. But I agree with the general frustration that it shouldn't be necessary to run apt(-get) autoremove frequently to keep /boot from filling up with old kernels.