That would be a bit complicated @Mittineague: if you have A-4 to A-6, they will appear on the same page, so you will read them sequentially.
Imagine we keep your example. As the first post in the list belongs to topic A, then this one will display first, and consume #4, #5, and #6. Next in line is topic B, with #12 and #13. After this comes #23 in topic C.
Next is either an alert: “there’s nothing new to read” maybe with “but there are N topics you didn’t read. [read them now]”, or a new cycle (you get back to the dashboard, and have a new cycle ready for you if you choose to do so.)
[Edited:] Note that categories would similarly take precedence over topics: if you have 3 unread topics in category X and 4 in category Y, the order would come from first-posted, followed by next-posted-in-the-same-category. That is to keep reading consistent. Then skipping a category might be a useful addition.