Thanks for sharing your thoughts here so much, Kayrenee! That really helps, and I feel better about kicking around some ideas here now. There are a few different options running through my head here for you to ponder. First, if you find separating them to be be something you'd consider, then I think that you have a good idea to do Beyond with your little one, and half-speed would be just fine, though you will probably want to do phonics/reading daily. We did half-speed with LHFHG and loved that. Then, we did the first part of Beyond half-speed too, and at that point my little guy was ready for full-speed in Beyond. That pacing worked well for him. You could just begin half-speed with your dd, and then either continue at that pace for several years, or go to full-speed at any point with her. I think she would love Beyond as much as our dc have.
For your 5th. grader, I think you could do PHFHG, and she'd probably enjoy that very much. It sounds like a good fit for her, and it is just an awesome overview of history to do. We loved Grandpa's Box especially and liked it paired with CHOW. It was great prep for CTC, and now CTC is smooth going because my ds has grown into being able to do the things independently he is supposed to in CTC. I don't think he could have done CTC last year. Since your dd is 11 yo, you could add in the Extension Package for her to do independently too. When you look at the titles for PHFHG's extension package, I think it's helpful to weigh whether a child can truly read the titles independently and do the suggested follow-ups on his own, since it is not meant to be another thing for us as parents to have to do. They are excellent books, but it's just something to consider. As far as the science, you could have her finish out CTC's, or if you aren't far into it, I'd really consider doing PHFHG's science instead. You could go either way on this really, we just enjoyed doing CTC's science along with the ancients history as it was just such a great meshing of history and science. The science in PHFHG is intentionally easier than CTC's science though, so you can weigh what you'd like to do there. We've been comfortable eliminating additional spelling in PHFHG. We did only dictation, and my ds (who had speech therapy for several years and initially struggled with spelling) has become an excellent speller overall. Once and awhile, like when writing an item on our grocery list for some odd reason, he spells things incorrectly, but most of the time mistakes are rare. He is really having to study hard now for dictation, but I think that's just making him do better and better with his spelling over time. Dictation has to be given a long while to truly make a difference though. So, you'd want to plan for it to take some real time to benefit dd's spelling in big ways. You'll know best what you want to do there, as it's easy to add spelling it that is what you'd like.
For your oldest, she could continue with Notgrass, and if that's going well, I'd probably keep doing that. Otherwise, CTC with extensions and additional LA/math could be used. I do think all 3 of them could do DITHOR together, and you'd just choose appropriate levels of books for each of them. That would be a neat together time.
O.k. - so that was sort of Option 1 I was thinking of. Option 2 would be if you really do want to combine, and that is first and foremost in your decision. If so, I'll be honest that there's quite a spread there in age, which makes it tough to combine with any program easily. If you were going to try to combine, there are 2 ideas I had - either to combine your middle dd with your older dd, or to combine her with your younger dd.
If you combine her with your older, you'd need to do it in CTC. Your older would do the Extension Package, and you'd know going into it that middle dd would need some help easing into it to be able to do it. She could do the right side of PHFHG for her LA/Math, so that would help with that in an easy way. You probably know if this would work well or not, as you know how these 2 would work together, and know as well if your dd would be able to grow into CTC quite smoothly. In this scenario, little dd would still do Beyond, probably half-speed.
Option 3 would be having oldest dd keep doing Notgrass or do CTC with extensions, etc., but combining instead middle dd with youngest dd. I am thinking that you could do this in Bigger Hearts. You could have the 2 of them do the left side of BHFHG together, as well as the Science and Storytime from the right side. Your middle dd could do the Extension package, which includes history and science titles. This would be a stretch for your youngest dd, so you'd need to know that going in and adjust as necessary to compensate for that. Youngest dd could still do the right side of Beyond for her Basics.
In all of the options, you could DITHOR with everyone, just picking appropriate books and using the Level 2/3 for your youngest, Level 4/5 for your middle dd, and Level 6/7/8 for your oldest.
My favorite option is the first one because I feel it meets each of your dc's needs so well. No one is stretched too far, nor is anyone going to need major tweaking to beef things up either. We did Beyond and PHFHG together last year, and found it to be very manageable. I have a toddler in the mix too, but you do have one older dd to consider as well.

Here are some links where ladies shared their schedules that may be helpful:
viewtopic.php?f=6&t=2093
viewtopic.php?f=6&t=95&st=0&sk=t&sd=a&start=15
Only you can know best what option would work best for your goals and in your home, but hopefully this gives you some things to ponder that may help you arrive at a plan that fits well for your family. Please continue to ask questions here until you have a peace about your decision. I'd love to hear any thoughts you have about these options after you've had some time to think if you get the chance. Praying for you to have clarity about placement.
In Christ,
Julie