I'm wondering which distro is a good foundation for a beginner, something where I can really learn the in and outs of the terminal and hopefully progress my knowledge?
You can use any of the major distribution to expand your linux knowledge. Debian and RHEL (or the free alternatives to RHEL: AlmaLinux and Rocky Linux) are one of the most used linux distributions. Ubuntu should be fine too. So, for server usage any of the four distributions are no bad choices. If you want to run a Linux distribution as a desktop operating system then you should check out Fedora or Ubuntu. Both are good for this use-case. Personaly I prefer the distributions from Red Hat (Fedora, RHEL (AlmaLinux, Rocky Linux) but as you may get the idea already, just try them and see what works for you the best. If you want to dive in into the core mechanics of an operating system you could check out Gentoo, Arch Linux or even Linux from Scratch (LFS), but the latter might be overwhelming for a beginner but you will learn the most by running & using especially the latter.