Linux

I would choose a popular distro instead of a niche one.

The main pro of that is that you'll get a big support community and lots of documentation. When you decide to use a less common one, especially one with only one or a few developers, this can backfire. Maintaining a distro is a lot of hard work and you have to trust them to patch all security-things and don't just stop sending out updates.

Most niche distros are just Debian/ Arch with some custom themes and a few apps pre-installed, which you can do on your own super easily anyway.

My recommendations would be:

• Fedora: my favourite distro. Gets major updates twice a year, and therefore is leading in new technologies and software, but not bleeding edge, so it is pretty reliable. There are many spins (editions) available, which differ in their desktop environment (DE), aka GUI and app family. The "Official" (Workstation) is with Gnome, which is a very unique and interesting DE that supports very smooth trackpad gestures (better than MacOS imo!) and looks very clean. But it is very unconventional and minimalist, so not everybody likes it. Hit me up if you have any questions. And the other one is the KDE spin, which looks exactly the same as your desktop mode in your Deck, since it also uses KDE Plasma as DE. Both are user friendly, but require something like a "10 things to do after installing"-tutorial, since there may be a few codecs or so missing.
• Mint: THE recommended distro for newcomers. It will take you by the hand and show you the wonderful Linux world. It's a bit more conservative, but therefore very stable. Even my Mum and tech-illiterate friends/ elderly understand it. If you like the UI of the steam deck, you may not enjoy it as much, but even when not, will introduce you
• Pop!OS: you already used it, nothing to say
• Kubuntu: similar to Fedora KDE, but not liked by the community as much. Would choose Fedora over it, but doesn't matter much tbh
• Fedora Silverblue/ Kionite: probably the most similar to the steam deck in this list. It is also immutable, so you can't brick it or fuck up much. But it should be more used by more experienced users.

--> I would recommend you Fedora Workstation if you have a good trackpad or prefer a keyboard based workflow. It would be a quite unique experience and something else than your Steam Deck. Otherwise, if your laptop is a bit older, try Mint.