Is entry level phones android 13+ less secure than mid range phones?
Or entry level phones android 13+ is still secure because of android as long as the boot loader is still lock?
Ideally you should be on the latest version of Android (currently 16, with 17 releasing soon). Older versions of Android will not receive all security updates, and therefor could be easier to compromise.
Yes, A locked boot loader is quite important to ensuring the integrity and security of your installed Android OS.
I do not know what your budget or threat model are, so I can not make any device recommendations, but I would recommend checking out: Operating Systems - Privacy Guides