MacBook Air2015-will be upgrading to newer machine: reduce space taken by obsolete docs, file

I've just manually transferred my data using the Finder since I have just a very simple setup as I don't have any music/video/photo/mail libraries to worry about. Most app settings I will recreate from scratch except for a few where I may manually transfer the .plist preference files since this gets rid of all the old stuff I may no longer be using. I tried to use Setup Assistant/Migration Assistant to just transfer my home user folder once long ago, but macOS still made some changes I was not expecting (I was trying to get away from a possible software & configuration issue at the time) so I just manually transferred everything I wanted. I know Setup Assistant/Migration Assistant do allow you to uncheck "settings" for both system wide & local user account which theoretically should allow you to decide if you want to reconfigure system & app settings or transfer them.

Carbon Copy Cloner (CCC) is a good option for copying & backing up your system since it allows you to select & deselect individual folders & files. CCC is a paid third party app which has been around for decades (wonderful developer....app used to be free until Apple kept making so many changes to macOS that the developer could no longer afford to keep it updated without charging a fee). SuperDuper is another third party app which also is well respected. I've always found CCC to be much easier to use, but both are great apps. FYI, don't bother with trying to make bootable clones using either app since that option is no longer guaranteed to work due to all the security changes Apple is making....it may still work for some, but others find bootable clones no longer work.....especially on the M-series Macs.

I'll have to leave it to other contributors to provide more specific details or instructions.

OmniDiskSweeper is a good app for locating the largest files & folders.

When moving to a new CPU architecture, I personally like to only transfer my data and manually download & install my third party apps. But this can be a bit too much for some people depending on their computer setup (my setup is very basic).

Make sure to purchase a Mac with more than enough memory & SSD storage which will work for your needs for at least the next five years or so since neither memory or the internal SSD is upgradeable after purchase.

Edit: And make sure to always have frequent & regular backups since there are a lot more new ways to permanently lose access to the data stored on the internal SSD due to all the hardware, software, and security changes.

Read Optimize storage space on your Mac - Apple Support and Free up storage space - Apple Support for some suggestions.

Rumors are just that. Rumors, gossip... waste of time. If you want to be informed attend WWDC. It's right there on Apple's front page.