Miniclient runs on very old flash version anyway, the reason you are black screening is not flash related, it's the login website being down on oasis end or something is blocking it on your end. I know some people in our group from Russia often have black screen issue and they need vpn to bypass it, because it just refuses to load from Russia but works fine if they use German or UK proxy.

As for flash support, that has an easy solution, just download old version of chrome or chrome based browser and don't update it, as long as you don't use it for anything else but only playing this game, you won't have any security risks.

And Flash support ending won't change anything(as i mentioned earlier miniclient actualy runs on very old flash version anyway that never gets updated, i think 16 or 21), i mean 10% of the world still use windows XP and support for that ended many years ago, it will still work just like it does now, just no more updates.