I had to make room on my MBP SSD to install windows through bootcamp and bashed my head against a wall on the same issue. It took me half an hour to find the reason why so much of my hard drive wasn't available despite me having deleted almost all third party apps and personal data on my macOS partition. Time Machine does 'local backups' and there is NOWHERE in the user interface that fully explains the space they occupy and how to get rid of it. To delete those local backups you need to use the terminal program tmutil.

That gave me even more vindication for my move. Also, you really don't know how fast your hardware is until you've used something other than macOS on it. From booting the system to launching software.. everything is snappier now.