One possible approach would be to expand Special:RecentChangesLinked with an additional option to show the changes to the associated talk/subject pages, maybe in a way similar to how Special:Watchlist works.

Could you please provide a link to "when using {{Special:RecentChangesLinked}}"?

Is this a request to allow filtering on RecentChanges to only display changes in a specific category? See T130134: Watching (only) categorization (additions and removals) of an arbitrary category.

Sure, here is a demo!

This request has little/nothing to do with the task you linked. It's about finding a way to display recent changes to pages whose talk pages have been added to a given category.

How do you add talk pages to a category without adding the corresponding content page also to the category?

I'm not sure I understand your question. Adding a talk page to a category does not cause or imply that the content page will be added too.

This is an interesting feature request and is related to some of the work the Growth team is thinking about for structured wiki projects. We will not work on it at this time but will keep an eye on this task.

I just found out that Massviews Analysis has a feature like the one requested here. Another way to fulfill this request would be to provide a way to generate a list of recent changes from the output of a Massviews Analysis query. Yet I think that a pure on-wiki solution would be better.

This would be very useful for a wikiproject standarization effort (with smart templates) I'm doing in the Spanish Wikipedia.

This is the only thing standing on the way of full wikiproject automatization on the Spanish Wikipedia.