Mac Pro late 2009 system storage

I have an old Mac Pro (late 2009) flashed to 5,1, which I use for music production.

I know this is a fairly old and common flaw with mac, but I hope someone has a trick for this.

I've installed an internal SSD which works absolutely great, except its starting to fill up from "System Storage". It's a 500GB drive, where 140GB is located as system storage. I've read a lot of different forums and watched countless youtube videos, but to no avail. I tried reinstalling from a time machine backup (I backup to an internal HDD) which reduced the system storage from 150GB to 140GB. I tried going through literarily every folder on the computer, to find the 140GB but I had no luck.

It seems like the system storage keeps piling up, until there is only around 5GB free space, when ever I delete files.

It's really annoying having payed for a 500GB drive only to have 140GB restricted from use.

Is there anyone out there, who have a possible solution?