What is a "must have" depends on your use case, personal preferences, and the shortcomings of your distro's default configuration (I've never used Cachy so I don't know what's missing).
For myself, I usually end up installing VLC and Strawberry Media Player, since the media players most distros come with aren't as good. On non-GNOME distros I tend to install GNOME Disks as it's the least painful to use of the GUI partitioning tools I have used. My preferred rich text format is Markdown, for which I use ghostwriter. I also usually install a few FOSS games to pass the time with - my favorites are Freedoom, SuperTux, SuperTuxKart, and Xonotic - and RetroArch for emulation.
Personally, I've had no problems whatsoever running the Office 365 apps needed by my school on Debian's version of Firefox ESR. Aside from Outlook and Teams, I'm not asked to use them very often, as most assignments are turned in as PDFs, but when I have been required to use Word and Excel, I have had no problems.
Apparently GNOME 46 introduced support for Microsoft 365 accounts including OneDrive support in the file manager, so a distro that runs a recent GNOME version, such as Fedora or Ubuntu, may be your best option. But without that, you can still use a third-party project like onedriver or abraunegg's OneDrive client.