As per the docs you link to, ExtendedSubCommandsKey should be a sub-key, not a value pointing to somewhere else in the registry.
If Explorer supports the value way of doing things, it is undocumented as far as we are aware, and this is the first time we've seen it. (Microsoft's docs on cascading menus very badly written, so they may well not reflect reality! Part of the reason we only have limited support for them, as the details are not really defined.)
[March 2020: I think the next paragraph may be out of date now, but I'm not 100% sure.]
(Also note that Opus only supports a single level of menus defined in this way, and won't work with nested versions. It doesn't look like that limitation should affect what you're doing, but it's worth knowing about if you plan to create menus via the regsitry. Discussion here.)
It's usually easier to create menus and sub-menus using Opus's own method, which can be done via the Settings > File Types GUI without editing the registry.