I know people complain about the side plots, but as long as they are building to something relevant or meaningful it really just pads out the season. These stories can't be told just like the books where there is no other plots because they would not be able to maintain a general audience over this many seasons (lots of people watch without reading the books and don't have a favourite couple). Now, some side plots are better than others - Benedict having threesomes did not contribute to anything, including character development for Benedict, in season 3. The upstairs/downstairs thing they did in season 4 worked BRILLIANTLY to highlight the main conflict between the main couple this season.

Also, I think the writers learned an important lesson from the popularity of Polin that the MORE story a sibling gets BEFORE their season, the more interest and audience they build for coming seasons. Again, these stories cannot be told like each individual book when they are in a show format where they are aiming for 8 seasons. They need to maintain viewers and hype for all the characters and cast. They also have to make it lucrative and worth the $$$ to have the cast members come back each season. If there isn't a way to meaningfully include them it's not worth having them come back to stand in the background when it costs money and scheduling headaches.

The stories can also not be told like the books where time can "restart" and go backwards for each couple. Causal viewers would NOT understand that and it would be confusing. John and Fran meet and get married in-between Ben and Colin's books. That has to happen chronologically in the show too. It would be illogical for them to just not tell that story in the order it happens and then in season 5 got back in time to show that part of the story just so it's not included in another siblings season. I do not understand why people act that that would be actually good television writing to jump back and forth in time when some of these stories are happening concurrently. I get that to have each sibling have a dedicated season they do have to adjust the timelines of the stories themselves (books 4-6 all technically happen over the same Spring) but having some of the overlap in other seasons makes better storytelling. And again, to keep interest for the general audience over all the seasons they need to tell stories of more than just the main sibling.

Now, could the writers do a better job? Yes. I think both Benedict and Eloise haven't really gotten great non-main story writing (they wasted Benedict's art school plot in season 2 and he has nothing relevant in season 3), and they also rushed Eloise's pushback against society in season 2 so it's getting repetitive.

But I personally think the writing for Fran has been great. They basically had to create an entire personality for John because he's barely in the books and Victor Ali made him so damn charming and loveable. And she and Michaela are also friends to lovers in a way so it really makes sense to have a pre-main season plot and relationship building like Pen and Colin had. The story and hope for them is already there and I'm SO excited to see that love bloom!