You don't specify how you are managing those mods. How you do so will determine the best way to back things up for use after you've reinstalled Windows and the game, etc.
Thanks to the Windows Registry and other unavoidable evils, there is no practical way to back up software that is actually INSTALLED into Windows. That is not true for "portable" software, which is fully independent and can be uninstalled and reinstalled simply by deleting and restoring their directories, but portable software for Windows remains a rarity, especially from big publishers. The problem with this: you can't back up software information locked away in the Registry (at least not trivially), and even when software uses independent configuration files they truly are "independent", scattered willy-nilly all over the freaking place depending upon the psychotic whims of the developer. Even aside from the horror of the Registry, trying to find and back up the state data of all installed software is a PITA.
So... you will be forced to reinstall and reconfigure Steam and whatever mod manager you are using, at the least. You could probably manage to avoid having to re-download all your Steam games, and there are several ways to do that: (1) use Steam itself to create a Library on the external drive and move everything to it, (2) move the entire SteamApps directory elsewhere yourself and then create an NTFS junction or symbolic link, where SteamApps used to be, to "point" to the new location. In either instance there would be a bit of extra work after you reinstall Steam to make it aware of the alternate location, either defining the Library location in Settings or deleting the default SteamApps directory and recreating the junction/symlink.
With respect to mods, I can't help you at all without knowing how you manage them. If you're not using Vortex, I probably still can't help much, as that is my current familiarity. Mods managed by Vortex can be backed-up fairly easily by saving certain Vortex sub-directories. The Vortex\downloads directory contains every mod you've ever downloaded (and not explicitly removed), and the Vortex\<gamename>\mods directories contain all the currently installed mods. It would probably be better to save the entire <gamename> directories, though, as those also contain other metadata you probably want to keep.
Reinstalling Windows is not at all a painless task, and the more software you have the more painful it gets. I have hundreds of applications installed! I've been carrying forward and upgrading the same install of Windows, for this reason, since the days of Windows XP. I'd sooner slit my wrists than have to reinstall AND reconfigure all of it!
Edited by VulcanTourist, 06 September 2020 - 10:50 AM.