Unfortunately, those two cases are precisely why you need to reboot. Linux will keep libraries and open files alive as long as there’s a process using them, which means updating a library will not come into effect until you restart everything that uses it. This also applies to open files, like data storage.
It’s not just going to be insecure: it might lead to data corruption or loss.
You could encrypt only your home volume instead of your entire storage; or you could switch to an “atomic” OS variant, which downloads and applies the updates while your session is running, and requires a reboot at your convenience without breaking your session. Another thing you can do is to favour containerised application mechanisms, like Flatpak, as they decouple OS updates from application updates, limiting the amount of updates that require a reboot.