Raspbery Pi OS Lite stuck on kernel 6.1

I have a Pi Zero W that was running running a Debian 11 version of Rapsberry PIOS Lite.  It was originally running Debian 10 and was successfully updated to 11 by an in-place update. The Bullseye update fine and with a few mods to minimise SDcard writes has been running for years.
A while back I upgraded it in in-place to Bookworm and it went OK. I think there were some strange network rules such that the Wifi is not managed by NetworkManager but it works fine. This Pi runs headless and is used as an SSH gateway from the wonderful internet to my network.
OK, I did an in-place upgrade which they say you should not do but it worked and the Pi is 100% fine. And I did an in-place upgrade again which worked. So that's 10->11->12
Except... I've noticed that the kernel is stuck on 6.1.x from 2023 and whilst there are updates for the components when I run the regular "sudo apt update / sudo apt upgrade" the kernel doesn't get touched. Looking on the PiOS website it's says Pi OS lite 32bit is on kernel 6.6.
So my question is whether there is some way with apt update/upgrade to get the kernel updated? Or I am facing (what I expect to the case) that I need to do a clean install of the latest image and re-apply my SDcard wear reducing mods?
Ideas?