Hi 10Newtons,
I have a similar situation to yours. I have three girls 14,12,10 and two boys 9 and 6, and two little girls 4 and 1. My older two are doing CTC, and my 10 and 9 yo are doing Bigger. I am planning to start the 6 and 4 yo in Little Hearts here in a few months, right now they are doing LHTH with the 6 yo doing phonics and math.
We have only been at this for about three months, since we started HOD in January. We are still getting the kinks worked out, but I will say that doing these two programs is definitely doable! I've found that the key is to have a routine/schedule, and to make sure that I get started at a decent time in the morning. I'm not a naturally structured person so I have to stay on top of myself to do this, and when I find myself relaxing then school ends up going on late in the day, which I don't like. So I make sure we get going in the morning and then we can have relaxed time in the afternoon to do other things.
CTC is mostly independent, I spend approx 40 minutes a day doing the teacher-directed activities with my girls. If things are crazy and I can't get to it, then they do it on their own. It's a four day program and they use the fifth day to do the science experiment, the poetry painting, and finishing up anything they didn't get done during the week. It's a great program and my girls are constantly telling me how much they like it.
Bigger requires the most time, but still ends up only being around 2 hours from me. Both my 10 and 9 yo need extra attention with language arts, so we spend the most time with that, I even do a little extra stuff with them because they need it. My 9 yo is not a great reader yet either and so I have to do everything with him except he can do his math mostly independently. We are really enjoying Bigger and it is a great program also.
My biggest challenges are keeping up with my 4 and 1 yo while all this is going on. I have my older girls take a couple of twenty-minute turns with the little ones, and that helps, but I don't want to take too much time away from their school, so the rest of the time I have to be creative. I try to get the 4 yo involved in quiet activities, but she is not a quiet little girl by nature

, so that doesn't always work. I try not to use the TV too much, so I get only about 30 minutes there. Sometimes I have one of the older three girls do LHTH with her and her brother, which she loves and so do they (they fight over who gets to do it with them

) The baby pretty much stays with me most of the time, she will spend a little time in her playpen with toys, or in her high chair with a snack. So the fact of the matter is, I can only do so much, real life happens around here and sometimes everything doesn't get done and I need to just relax about it. If I stick to my two keys of structure and starting on time, most things do get done, and what doesn't we get to when we get to. It doesn't have to be perfect (at least I hope not

)
As far as getting your oldest ready for high school, I hear you there. What exactly are your concerns? Are you planning on a public school for her or high-school level work at home? For my oldest, she is technically high school now, but she wasn't ready for true high school work, an we have been focusing on love of learning for her, which HOD has really done. To be ready for high school academically is primarily about math, writing skills, and starting in the sciences. You can focus on advancing with these things whichever program you are using. We are trying to get my daughter a solid foundation in math right now, and working on her writing skills, and I am going to ease her into Apologia science when she and I feel she is ready. I am not worried about her getting where she needs to be. She will blossom when she is ready and advance quickly to where she needs to be. So I would say if your dd is motivated and wants to advance quickly through ctc and rtr that is great, but don't feel like you need to push her. These programs are tools for us to use to help our dc grow and learn, and so use them the way you need to for your childrens' greatest benefit. Steady progress and encouragement combined with a love of learning will get us where we need to be - that is my philosphy. Anyway, I don't know exactly what your concerns are with your oldest but that is what is going on here. Let me know if you have other questions! I've got to go get the morning started
Hugs!