Distro choice matters less than it looks like, and it's fairly subjective. As long as you stick to a serious and newbie-friendly distro, you should be fine - for example, you could simply keep using Pop!_OS, why change it?
That said, a few distros you might want to try:
- Mint - another newbie-friendly Ubuntu derivative. If you feel like you must try something else, but you don't want it to be too far from your comfort zone.
- Debian - because it's the grandfather of Pop!_OS (and Mint); it has some rough edges, but it'll be a good learning experience. Note Stable tends to stick to really ancient packages.
- Fedora - it's also newbie-friendly, but from another different family. If you feel like stepping outside your comfort zone.
Also note you can dual boot different Linux distros, just like you're dual booting Pop!_OS and Windows. Or even multi-boot.