Reading your post, I made a long list of suggestions for you, some made by others in this thread:
• Look up the list of Big Sur supported macs on Mac Rumors. Big Sur may be the last Apple operating system for MacBooks, Mac minis or iMacs with Intel processors. Personally, I wouldn’t spend the money on a brand new machine, I’d wait for the Apple Silicon processors coming in 2021/2022 before buying new.
• Search for sellers of used Macs and buy something 2013 or newer - again checking a list of Mojave, Catalina and Big Sur supported machines. I’ve used Mac Of All Trades several times (I do not work for them) and they give you money for your old Mac. There are other companies that do the same thing.
• Keep your 2011 Mac mini and upgrade to a Solid State Drive (SSD) as others have suggested and stay on High Sierra. Purchase VMWare Fusion or Parallels, as others have suggested, and run a new MacOS in a virtual machine. I’d suggest increasing the RAM to 16GB if you do this . You can replace the RAM yourself or have it done with the hard drive upgrade. Increasing the RAM will allow your virtual machines to have more memory without starving High Sierra. I have a 2012 Mac mini running Catalina/Big Sur Beta as a dual boot virtual machine with 4GBs of RAM allotted to the vm out of 16GB. leaving 12GB for Mojave on the Solid State Hard Drive.
• Do any web work/surfing in the virtual machine. Stop getting on the internet with High Sierra. Make sure the firewall is on on any Mac you own. Get a good virus scanning software that runs in the background, Malwarebytes, ClamXAV, etc. so your Mac is constantly monitored. This will check web downloads and incoming mail attachments as well as the rest of your Mac. Purchase software like Little Snitch that monitors outgoing traffic from your Mac - firewalls and virus scans handle incoming traffic not outgoing. You’d be surprised how many applications phone home - looking at you Adobe : ) This will not impact your security but helps with privacy.
• Look into various web browsers re: security features - Safari is not necessarily the best. Check out Firefox, Brave, etc. Your choice may allow you to surf from High Sierra with minimized risks. And these browsers could be used in the virtual machine too.
• Take snapshots with the virtual machine software on a regular basis. This acts like Time Machine, keeping a record of your vm at a given time. If you get something bad in the vm you can go back to a time before the incident. Worst case, you can trash the virtual machine and start over without affecting your High Sierra installation.
• And speaking of Time Machine, backups of your High Sierra hard drive. Some choices for third party software are available for cloning. Again, going back to a time before a security breach.
• Use a patcher like DosDude1’s to upgrade your 2011 Mac mini to Mojave or Catalina. Caveat: these patched OSs don’t play well with the AMD Radeon version of the 2011 Mac mini. The patchers work well with the Intel 3000 graphics. I have used the patches on an early 2009 and two Late 2009 Mac minis to install Mojave and it runs great. There’s a lot of information on the patchers here in the MacRumors forum - check out whichever os on unsupported Macs in the forums.
• Consider a Virtual Private Network, VPN, for web surfing if you want to retain more privacy. Again there are many software/service choices out there with a lot of review to help you decide.
• And finally, as you know, don’t click on links in emails or on web pages. Probably the greatest cause of security problems. I have a 2005 G4 Mac mini that I sometimes surf the web with. It runs OSX 10.4.11 Tiger, unsupported by Apple for years. I use TenFourFox browser, a port of Firefox and never download anything with it. 15 years later, no viruses.
