The simple answer is that you have installed server users and databases on your Mac so you might want to restore from a backup if you have lots of other apps you don't want to reinstall over a clean OS Install.
The clean install is also viable - be sure to only restore your files and settings - not other users or applications from your backup since the Postgres database could be installed in many locations.
The most surgical option would be to find the specific uninstaller script for the specific version / flavor / edition of the software. Lightspeed POS has Lightspeed Retail / Lightspeed Restaurant / Lightspeed Onsite /Lightspeed eCom - each of which has a couple of variants, each of which has dozens of versions.
The vendor made a script to make all the changes to your system, so with a product like this, the chances they have a specific uninstaller for your specific situation is highly likely.