If I bite the bullet and install 10, will I have to uninstall and reinstall all my sims and addons?

Yes, because I would not recommend just upgrading to Win10. Clean slate, bro.

-Ice is correct,,Clean install prevents gremlins,,,Just be sure you have backups of installers mods patches ETC....I found I could just zip and back up most sims and unzip them into a fresh install stuff like IL-2 SF2 FSX and with Steam a lot of my games are just a fresh download away..It's a job but I learned I don't have to have everything back on the same day I did a couple a day and had a smooth running setup but that was a fresh Win 7 install when things started getting buggy The advice is also great for Win 10

If these are steam games, keep them on a gaming drive or move to one. After installing your OS, reinstall Steam in the same directory overwriting itself so it can write into your registry. Or if a clean steam install, direct it to where the games are as the install directory. Some games may need a registry entry too, but you can accomplish that just by having steam verify the files.

Upgrade installs are much more robust then they used to be so I would try that first. I'm on a Win8.1 upgrade install and no issues for five years or more.