TL; DR: From personal experience as a Raspberry Pi tinkerer and Windows evacuee, I recommend Linux Mint.
Raspberry Pi OS is essentially Debian compiled for ARM with the LXDE desktop. They used to use LXDE, and it is my understanding they forked LXDE to make their “Pixel” desktop. Being Debian, it uses the APT package manager with .deb packages.
Linux Mint is a fork of Ubuntu, which itself is a fork of Debian. It uses the APT package manager and .deb packages. The exact same commands to install, say, LibreOffice on a Raspberry Pi can be used to install it on Linux Mint.
Cinnamon is the flagship desktop, and I think is a reasonable answer to “What if Microsoft had kept developing the Windows 7 desktop instead of trying to make a tablet OS?” I chose Cinnamon pretty immediately because it felt more like the Windows I had grown up with than Windows 8.1 did.