I did exactly what you are doing, with 2018 iMac running Mojave. Installed first on external SSD to see what it looks like, migrating just a few apps . All good so I Then installed Monterey directly to my iMac via software update. Again, very smooth, no hiccups. Unlike some others on this list, it installed without hours of delay. I had previously done the cleanup via terminal described here Explanation for some Big Sur upgrade problems.