Except when I used the 3rd party app the computer doesn’t lock up and need a hard power off to get restarted.
That sounds like you have a software interference at play, not an OS issue or RAM issue. Any anti-virus or so-called "cleaning" apps can do that.
And please— share the identify of the "third-party app." The senior contributors here have a long history with most.
And, as Grant points out, Memory Used ceased to be a viable metric for Macs in 2013. The current indicators are Memory Pressure and Swap Used. Both can be viewed with the built-in utility Activity Monitor. Here are memory use data for my older iMac after I installed macOS 10.9:

After viewing Memory Used, notice:
- Pressure is as low as can be displayed
- Swap is ZERO.
What you cannot see is that the computer performance was absolutely normal and stable with those numbers showing. That computer has never had any useless third-party cleaning or antivirus apps installed.