What if, instead of combining them, you increased their independent work?
My oldest is 10 doing CTC (has been through all the other guides), and was in Preparing last year when *I* was pg with number 5 with a 3yo in the mix

I had added in her reading her own history fully about a month before the baby was born (started out with her reading it one day per week, then two, etc) It was amazing how much that helped me. I also started reading the Story Time books at bedtime. So she was independent in History, science, independent history, vocab (because we worked on that through Bigger, more to come on that), poetry (because I needed her to be), timeline, a lot of the Bible, and English. After she finished all that, she would come to me (or obviously if she had questions)... I would also teach her the math before she went to do her work, so she did the workbook during her independent work time.
My second is currently almost 9 and in Bigger. We have worked her up to where in vocabulary she can find the definition and copy it by herself and draw her picture, but she needs a little help with the sentence (her brain does not work that way). In math I teach it and she works independently. We are currently working on her taking over her science and/or history. Today she chose to read her science. Surprise of all surprises she picks the shortest one every day

We are also at the point that with her notebooking I sit there while she reads me the instructions and then she goes and does the work, both science and history.
I cannot feasibly combine mine right now... Grace remembers all we've done in Beyond and so far in Bigger, so I can't put her back down with Julianna. Jayden is way past them, so I can't move Grace up. Julianna could easily do Bigger, but it doesn't seem right to jump her from the first 1/3 of Beyond to the middle of Bigger.... a lot of stuff is in there! And Evie and Penelope aren't nearly to the level the big girls are. So those are my suggestions based on my personal experience.
My only other thought would be to back off on Bigger to half speed or 3-4 days per week.
Oh, and in the 3-4 months leading up to Penelope being born, I spent a LOT of time working on chores/housework/jobs. My oldest three can do pretty well everything I can do around a house. All I have to do is tell them what to do and they are able to do it.