How do I delete files on my MacBook Pro for significantly more space?

I would get a nice large external drive which you can use for a backup using TimeMachine! Make sure you write down any software keys if you have apps other than what you downloaded from the Apple Apps Store.

Do keep in mind Catalina or Big Sur don’t support 32bit apps! So you won’t be able to run them after you upgrade. So before you upgrade make sure you're clean.

If you have a smaller drive (256GB) it maybe time to upgrade it so you can have the needed extra space. One of the biggest issues I see is people running to lean! That is using a system who’s SSD drive does not have enough free space. A 256GB drive needs 1/3 of the drive unused, a 512GB 1/4 a 1 or 2TB drives can get way with less.

In any case once you’ve backed up you can then reformat your system and install a fresh copy of macOS that you want. Then at the end of the Install process you can transfer your TimeMachine backup using Migration Assistant. You can also be selective! moving your user accounts and Apps leaving your data on the drive.

When you install the fresh copy of MacOS you can go into the advanced tab deselecting things you don’t need as well like language and fonts.