This might not be your issue, but I remember for me there's this button in the tray that appears after all updates are downloaded. You click it and there's an option to restart and update. If you just go to the main menu to restart, it doesn't always show this "restart and update" option, and just restarts without installing the updates. Though even if you make this mistake you shouldn't need to re-download the updates. Just go to KDE Discover and check for updates again, and the button will appear in the tray again. I think.

Also maybe you'd like the Fedora atomic distros, like Bazzite or Aurora. Unlike installing the updates during reboot, they install them in a "disabled state" while the system is running. Then you reboot and it automatically boots to the new version, no need to wait for any installation. What's neat is that it's really hard to break. Even if you pull the power while it's installing updates, it will just use the previous version and try installing again. No risk of corruption.