Welcome! Some of my must-have FOSS software for GNU/Linux are:
ONLYOFFICE: Similar functionality to that of MS Office, but free and open-source, very nice compatibility with .docx documents and all the excel formulas I use are still there.
Boxes: If you like or need virtual machines, Boxes is one of the best FOSS solutions out there, I have made Windows, BSD and Linux virtual machines using Boxes and they work flawlessly, and the drag-drop feature to send files from the host to the guest machines is absolutely nice.
Konversation: In my opinion the best graphical IRC client, with HexChat also worth noting.
Kdenlive: I have used many video editors in my life, both FOSS and proprietary, but Kdenlive is the one who made me stay. I have even remastered old 80s Betamax videos using only Kdenlive.
TeXstudio: If you like LaTeX, this editor is absolutely wonderful and it works out of the box.
Prism Launcher: If you like Minecraft, this is the only launcher that actually worked on my Fedora installation, and it’s so easy to install mods, resource packs, shaders, etc. that I already consider it to be the best FOSS launcher for both premium and non-premium instances.
HandBrake: I just love this open-source video transcoder so much.
fre:ac: I have used this FOSS audio encoder since I was a kid when I wanted to convert mp3 music to a format that my DSi could read. Nowadays I still use it to convert from and to any type of audio and it just never fails.
RaccoonLock: A modern-looking and private password manager that is wonderful if you just want to store your passwords locally in your PC and you do not care about syncing them with other devices (although such feature is partially possible through the creation of backups).
It’s also worth mentioning other FOSS software like VLC, VS Code (though it’s not entirely FOSS, with Codium being an actual FOSS version), OBS Studio, GParted, PDF Mix Tool and FreeTube. Welcome to the GNU/Linux world! I hope you enjoy it and you find these utilities useful :).