Congratulations SueDenim!
I can't believe I came in second with only a week and a half - I guess every one was super busy this month. I know I made quite a lot of mistakes in that week & a half too since I didn't really practice either.
I did spend quite a bit of time thinking about how to approach this. In particular, I thought a lot about how I could overlap sims completing the love aspirations as much as possible. I mentally weighed up whether the extra point from 1 extra serial romantic would be worth the extra time needed (compared to having 2 soulmates in the same household). In the end I didn't get to practice any of my theorising so I decided to just go with the following plan - 2 sims in each household would do soulmate and everyone else would do serial romantic.
All dates were done at home. I aimed to micromanage things so that the serial romantics would practice charisma while the soulmates got through the first couple of levels of their aspirations. Since I mostly focused on households with an already married couple in them, that was usually done in a few hours. Then the soulmates were left to their own devices for a bit while the serial romantics zipped through their initial dates.
You can cancel the initial dates for stage 1 of the love aspirations straight away without much penalty. (Seems to be just a low chance of an awkward sentiment). I usually did this for the soulmates except sometimes I forgot. But since the serial romantics' later stages would need them to have had a certain number of first kisses and boyfriends/girlfriends, I would make their initial dates go until they got to that point.
Once the serial romantics had done their initial dates, they would work on all the kissing they needed to do while the soulmates did their 3 gold star dates needed for the final stage of soulmate. Then the serial romantics had to do their gold star dates and occasionally I'd queue up a pile of romantic socials for the soulmates to do with each other so that they would also finish. The soulmates generally added a few romantic interactions to their counters autonomously but not as much as I'd hoped.
I found ways to keep the soulmates away from the others so that they wouldn't interrupt conversations if they ran out of queued tasks as generally the serial romantics needed my attention more. Either I took advantage of the door locking feature to lock them into the master suite or I put them upstairs and then moved furniture in the way so that they couldn't get to the stairs anymore.
I also used clubs a lot, mainly for my serial romantics. It appears to be the quickest way to meet sims and you can make your next prospect turn up in the middle of the night, if necessary. I made sure to pass the clubs onto the next household before moving to avoid having to save up for perks again. The soulmates would only ever join a club very briefly when we needed to rally the troops.
Clubs or the "add to group" function were also used frequently to move a serial romantic and their next romantic propect into a different room/inside/outside/anywhere where they couldn't be seen by their previous romantic prospect, if the sim hadn't got around to leaving the premise yet despite me sending them home. I also made sure that at least one of the early dates was gold to get the reward which gives a flirty aura. If necessary, I would move it round the lot to wherever it was needed but usually between all the early dates I ended up with more than one. If it wasn't enough to change a romantic prospect's mood to flirty on its own, I would make use of brighten day/heartfelt compliment and/or taking photos with them to give them enough happy moodlets to boost flirty above any negative emotion.
One of the things that I could have done a lot better if I'd actually practiced was my choice of sims and households. I definitely made quite a few errors there. eg Rohan Elderberry's loner trait makes him not a great choice for serial romantic. I probably should have gone somewhere else rather than the Elderberrys at that point but by the time I'd discovered that, it was too late and I had to make the best of it (extra charisma training, extra flirty moodlets, and - unlike the rest of my serial romantics - I tried to have him avoid romancing anyone who I knew would have some romantic feelings for other sims already ie. all the teens who'd been involved with Olivia Kim Lewis at the previous house).
The first household I went to after my starting sim was the Spencer-Kim-Lewis household. My reasons for picking it were mostly rendered moot by the fact I only managed to complete a week and half but I'll explain my reasoning anyway. Going there early meant I would get their household done before there's any chance that Vivian or Dennis might die and Vivian also starts with some cooking skill. So while Alice & Eric got started on soulmates and Dennis practised charisma, Vivian baked a couple of birthday cakes and Olivia used a club to get groups of children to come over. They were all made to age up (Olivia first) thus providing plenty of suitable romance prospects for my teen serial romantics.
No one ever went to work or school either. I just had them call up and take vacation days and I wasn't ever with a household long enough to run out.
That's about all I can think of. The almost constant dates started to get to me after a while, so I ended up deciding to finish after my 5th household (that's including my starting household). There were only a few hours left before the deadline here at that point anyway - I probably wouldn't have managed another household anyway, not with how slow I tend to play.