This article is typical of what I've seen: How to Play Old Flash Games in 2020, and Beyond
It, like all the others that I've read, recommends Flashpoint.
From the Flashpoint FAQ:
Does Flashpoint pose any security concerns?
The entirety of our application suite (minus the tech needed to download games via Infinity) can't connect to the real internet in and of themselves. Any changes made with Flashpoint Secure Player are removed once the game is done being played. BlueMaxima has been working with these files on his hard drive for some time, with competent and up-to-date antivirus software. We've done our best to ensure the collection is as safe as it can be. It is worth noting that some antiviruses will detect certain files as being dangerous; in every situation we've come across, this is a false positive.
I haven't used Flashpoint but maybe somebody that has will comment on the functionality.