The journal is definitely a good place to start. To keep it from growing too large you can follow this tip from the community wiki: Tips & Tricks · Wiki · Librem5 / Librem 5 Community Wiki · GitLab
I also noticed that the cache in Firefox had grown large as well, even though I had set it to clear every time it closes. That could have been because I have have some websites I set exceptions for.
One last tip is flatpaks, if you have any apps installed. Byzantium by default has an older version of flatpak which has a bug where it keeps older unused runtimes. To clear them out you can follow this: Low disk space - how to clean out garbage? - #54 by wimdows
To update the version of flatpak you can use this to help guide you: Flatpak Bug Librem 5 - #2 by Privacy2