Ignore “Cached Files” that’s just a benefit optimization if the system has extra RAM to play with.
As to why I said 8GB would be tight and the mini would "be throwing down at least some swap”...
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The latest version of Catalina (10.15.7) running Activity Monitor, Mail, Messages, Safari (only a single tab : this page) as well as Affinity Photo (1.9.0) with a 4032x3024 photo open.
I’m not aware of the exact threshold, but the system will create swap files before completely filling RAM.
Ultimately, would 8GB be unusable or devastating? No. Nonetheless, I strongly recommend more for your use case.