mkl The /boot partition does not mount at system startup (that's the case in my situation). Unless you mounted it manually later. Therefore, there shouldn't be any issues performing operations like resizing this partition. However, if any problems arise, then yes, you should definitely use Solus Live.
snowee As I mentioned earlier, any operation on system files, including the /boot partition, can lead to system corruption, so I recommend extra caution.
I removed unnecessary entries in /boot/EFI/com.solus-project/, I ran sudo clr-boot-manager update, and restarted. Everything is fine.