I think a first step should be to create a custom collection for $allcontent. That way, you can easily pass this collection to the feed options.
Please rename your variables. Dashes are not allowed in PHP variable names. In the Kirby context, we use camelCase for readability, but you might as well use underscores to separate words/parts.
As regards combining multiple fields into one, you can use a custom page method or a page model that returns what you need.