Four Mac Mini's (2014) with Software Issues

Greetings everybody,

I am facing difficulties with various software and performance issues and I would greatly appreciate any help suggested. The main tasks we use these four machines is to perform video rendering, transcriptions, and to access a shared folder to drop content into through a Windows workgroup running Windows 7. The first three machines are running High Sierra and the fourth machine is running Sierra. We updated the the first three machines to High Sierra mainly due to server sharing issues that we were facing now and then. Here is a complete list of the issues we have been facing:

System 1
- Does not connect to internet at all after High Sierra update.- App Store is blank, does not load, probably due to the first issue- Not able to see all the files and folders which are existing in a certain Shared folder, which others can see fine who are accessing the same Shared folder of a different computerSystem 2
- QuickTime hangs (plays video slower than it's normal speed, audio mutes and when pause button is pressed, it pauses after a while)- When any other program (BlackMagic or Hedge) is running, it seems like a deadlock situation (the processes don't get enough memory so that they can run/hangs), like it's running on a slow system [Happening on System 1 as well]- In Numbers, when we save the file, sometimes we get a message that, "The file does not exist and cannot be found in the source folder." (this is not the exact message, but this is what it conveys) [Happened on System 1 once, Happened on System 2 twice). On closing the file, the file is deleted forever and nowhere to be found- Sometimes when starting up the system when power button is pressed, the load bar with Apple on screen looks like it is progressing, but it never finishes. After one or two hard boots, the system starts up fine.[One time it was doing that for 30 minutes. It reached the end after 30 minutes, but nothing happened after that. After 2 hard boots, it was working fine, with a 5 minute startup time ~ this happened on System 1]System 3
- On one page in Safari, the mouse froze and nothing else responded. After a hard boot, everything worked fine. Safari reopened to same page, mouse froze up again and nothing responded, same issue as before. Did another hard boot and closed Safari without navigating in that page. It was fine after that.System 3 and System 4
We have not been actively using System 4 at all. System 3 we used only for a few hours on one day. System 1 and System 2 have been actively in use for one month.All System Issues:- Permission Issues:

• There is one Mac Pro with a shared folder. Users from these MiniMacs are not able to access folders that Mac Pro user are creating, even with Read and Write enabled for their login. All Sharing is set up properly with a separate login for each user.
• Parental Controls,
• When enabled, despite ticking off permission for individual Applications, it asks for Administrator password every time, even after selecting Allow Always option.
• Despite changing settings in Parental Controls, it doesn't reflect even after clicking the lock sign. Tried to change it three times, to no avail. Then it showed on the fourth time, but did not reflect the settings when accessing applications.
• Then when switching off parental controls, it was still asking for Admin password for all applications for sometime. Only after clicking OK continuously, instead of allow once or allow always, it stopped popping up.

Overall, the systems also feel slow.

For example, System 3 is noticeably faster than other systems. Even when it comes to USB recognition time, System 3 pops up immediately, the other ones take longer. System 4 is yet to be tested.

Thanks in advance,