I've become a rotational player with TS4. I've always been a family legacy player. With TS2 I was too young to play long enough so I could have my Sims' kids have kids which would have resulted in rotational play in the end, my parents wouldn't let me play a lot. But I do remember playing 2 families rotationally once because I wanted their kids to marry one day (I think it never came to be). With TS3 I didn't play rotationally, I had a story progression mod which made everyone I didn't follow live their lives, so I picked an heir in my family to play them actively. But we all know the troubles with TS3: when you played a save for too long, it used to blow up and became unplayable, so I think I made 2nd generation once, but I never made it beyond that.
So even though I miss open world and I am sometimes annoyed by all the loading screens, I am happy the game is much more stable with TS4. The 2nd generation has just been born and my game runs smoothly. I'm an adult now so I can play as long as I want and I've been playing my current save since 2017 (with long breaks in between).
Now to your question:
1. I change households at 5 AM (like everyone else here, haha) so my Sims won't miss work or school. I usually play a family for 4 days (did 3 days up until a little while ago, but I noticed it was just too short to really develop their story). I try to be very strict about the time I play each family because I don't want them to age differently as I play with aging off except active household. I randomize the amount of kids my Sims will have, otherwise I would just let everyone have 5 kids or something and I'd end up with too many Sims. I also randomize their traits and aspiration and then I create a story along the randomized traits and aspiration (otherwise I'd end up with the same traits and aspirations all the time). I give them a new every day outfit with every seasonal change when I return to the family.
2. Right now I have 5 families: the legacy founder couple (who are old and my dear Sim will die soon) and their four kids who have already moved out, 2 of them have kids of their own already. So I've got 12 Sims in total right now, pets not counted. 3 seniors, 3 adults, 3 young adults, 1 child, 1 toddler, 1 baby. Another baby will follow soon. :)
3. As I said: I've been playing this save since 2017 with long breaks in between. It's not that I lose interest but I always think playing Sims consumes so much time and when I work or have much to do academically, I won't have that much time left for playing, so I often rather stop completely than just settle with an hour a day or something (you can hardly do anything in an hour of sims gameplay and it's not that immersive). I am really enjoying the different storylines right now, it's become more fun with more gameplay options of course because you can do different things. With TS2 and TS3 I hardly got to try all of the stuff the game offered because of lack of time and blown up save files, TS4 doesn't offer as much and I wish it would offer more. But it is enough for me right now as there are a lot of things I haven't played (probably won't ever play Vampires and Stranger Ville, but apart from that there is so much I still haven't done).
4. I tend to fill up my neighbourhoods with premade houses I download from speed builders (love watching speed builds). I usually change them to fit my needs and taste. But as this takes a lot of time and I don't want to JUST build, I tend to build between family rotation for a little while, so my neighbourhoods are a mess right now. In Newcrest I have a church, lots of community lots (also a school on which I am working right now, I play with Zerbu's school mod) and as for now only 1 family home for one of my families (others will follow though).
5. Yes, there are drawbacks. You can set seasons to 28 days the longest. I wish I could play my families 5 days each and not 4, but I want everyone to experience every season and it won't work so well anymore with 5 families and just 28 days. I would have to give them new outfits everytime I change family and that is a bit tedious. Otherwise there is gameplay for which you need more than 4 Sim days and which will not freeze when you change family. For example the actor career or University... I don't know yet if the actor career even is playable for me whith my playstyle... Relationship culling would be another drawback, but I use a mod to prevent that (it still happens sometimes though, it's really annoying).
6. As said, I am a family legacy player. I usually start with a couple I made in CAS, sometimes they already have kids (in my current save I made them toddler twins), sometimes their kids are born in game shortly after. My first family will always have lots of kids, 3 at least, but most of the time it's 4-8. In this save it was 4, but I would have loved more haha, love playing with families.