There really isn't any popular alternative to Flash today, and I think that's kind of a bummer.
WASM is looking increasingly good these days.
Have a look at egui for instance, and just see how fluid and perfomant it is on all platforms - and that is running without using any insecure/clunky/buggy plugins.
The only issue (with egui) is that it's basically Rust so it's not exactly newbie friendly, but that's just a tooling issue. Hopefully in time we can get more newbie friendly tools, and with increasing number of apps using HTML these days, we might just see something as easy to use as Flash soon enough. :)