Before you upgrade from High Sierra to Monterey, you might consider installing the free Go64 application that will list all 32-bit applications and library dependencies you currently have installed. These are not supported, or will require product upgrades with macOS Catalina or later.
Next, perform a last Time Machine backup of your High Sierra installation and then eject that drive and set it aside labeling it with the backup date/time and High Sierra 10.13.6. You should use a new Time Machine drive formatted as APFS via Disk Utility for Time Machine backups after the operating system upgrade. Time Machine will expect this formatting. You will need to assist Time Machine in recognizing this new backup drive.
After upgrading to Monterey, the first launch of Apple Mail will automatically migrate your email to the version of Mail in Monterey. This is usually seamless.
You should also install the current Firefox or Brave browser after the upgrade to maintain security currency with websites. Safari will be current enough at this point.