Which shell to use to modify/replace Windows Explorer (don’t want fancy desktop but better browsing of folders/files... similar to Classic Shell)

You want an alternative file manager, also known as an Explorer replacement. Some of them will take over folder open operations, so that double clicking folders opens the file manager. e.g. Directory Opus and xplorer2 are two I know of that do this, there are probably others.

However, shell dialogs in programs (Open/Save) will still be driven by Explorer. It's really more accurate to say that Explorer is a relatively thin veneer over the shell's file management abilities. It also provides the Desktop and Taskbar, but the file management abilities mostly come from other components in the system. Explorer doesn't provide them to the rest of the system.

That's why they're so difficult to override; to heavily modify those components requires messing about very deep in Windows with ramifications for every non-trivial application. Especially since those DLLs get loaded into the memory space of those processes, screwing up a mod potentially destabilizes almost every user-facing application.

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Try Explorer++, light and fast, does not install anything unless you configure it to.

you can install TeraCopy to replace the file copy/move functions.