We have just engaged in the Windows 10 migration process and I am looking to run 1903 as even that is getting superseded in December 2020 so only really getting a year out of it, when you have 3500 machines to update a 1 year build cycle is really small! Working on a more streamlined deployment process but that is going to be a while in the works.
The application works once you accept to continue to the login but I know people will flap if it throws warnings, we all know what users are like when something looks different.
Yeah, flash is well and truly dead, people should really be moving to HTML5 by now.