Yes previously I have sometimes copied the .dll files from the Qt library together with the dynamic Qt build and it has worked.
Are you aware that Qt comes with a deployment tool that identifies the DLLs required for a dynamically-linked distribution?

with Qt5 ... this did not work any more.
Because in Qt5, the dependence on the underlying OS and GUI system was refactored, which resulted in a different directory structure required for the installation of the executable. An install now requires a "platforms" subdirectory under the directory for the executable that contains the platform-specific DLL, in this case "qwindows.dll". If you have other plugins, like database drivers, these have to go into a "plugins" subdirectory, and so forth.

From what I read of Qt6, more refactoring has occurred with respect to the platform's graphics system, but i do not know if this has changed the deployment requirements. Likely there are more platform-specific DLLs required.

a simple win32 hello world static Qt5 application takes 18 MB
A dynamically-linked Qt5 Windows app takes the same. Qt5Core, Qt5Gui, Qt5Widgets, and qwindows DLLs add up to 18 MB. Apparently your linker drags in the entire static library regardless of how much of it your app actually uses.

and needs more that 10 floppy disks to be distributed.
Ummm, there are these new shiny things called "CDs" and "DVDs" that companies use to distribute software now. Some of them are even using this invisible thing called "the Internet", which sort of works like Compuserve, except without wires or a 1200 baud modem. You should look into it.