Perhaps I'm misunderstand you but Catalina is in my System Preferences and not the App Store and I don't remember seeing OS updates in Settings until I went to Mojave. Prior to that it was lumped in the App Store with everything else. I don't personally have an issue with that except I don't want it constantly nagging me to upgrade.
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I am not getting nagged that much. Also, if you do a search of Catalina in the App Store, you will find it (but for me, I wanted the full installation Catalina file, which is what the search came up with).
Also, I have done a clean, fresh installation of Catalina on an external SSD, and then migrated/copied needed items from a SuperDuper! backup. Restarted my Mac from it, and it was fine. Except for some (expected) 32 bit apps being incompatible (it also flagged some that I no longer need/use), just about all my third party apps work fine: VLC, Thunderbird, LibreOffice, Google Chrome, FireFox, Opera, Quicken 2017, Transmission, EasyFind, AppCleaner, TrashIt!, ClamXAv, and 1Password. And my Canon printer works fine with the printer drivers built into Catalina. But as expected, SuperDuper! and TechTool Pro are not yet compatible with Catalina. (There is a recently released second beta of SuperDuper! V3.3, but I am not going to mess with a beta release). Finally, Titanium Software did release V3.7 of Onyx the other day (Catalina version), but I have not tried it yet. Expect it works, though.
I'm sticking with Mojave, most likely for at the least the next 2 months, as that should be when a Catalina version of TechTool Pro will be released. And if it's anything like it was with Mojave, that will be after V10.15.2 of Catalina is released, and maybe just before V10.15.3. That is what happened earlier this year, as TTPro was released for Mojave compatibility in January, and OS 10.14.3 was released only a week later.