Gwenny,
As far as your situation goes, I would encourage you to choose one set of story time books to read aloud to the Beyond/Bigger kiddos (and follow the plans for the younger guide, so you can keep this pattern going as they head into the next guide too). I would also work toward getting the kiddos who are the most ready to read from their guide to do so as soon and as much as they are able.
This doesn't always mean this will be the older child either!
If your child eventually heading into Preparing is able to read his/her own history, I would encourage you to let that child do so. Or, perhaps your first Bigger student might be able to read the science (or possibly even the history). I did allow my second son to do this as he was ready in Bigger. My next little ones in Bigger could not do that though, so I read everything aloud to them.
If you have a child in the Emerging Reader Set, it is so helpful to have that child practice their day's pages alone first before coming to read to you! This allows the child time to peruse the pictures, figure out difficult words, and ruminate on the story. Then, by the time he/she comes to read to you the reading clips along more quickly and the follow-up questions are more easily answered.
Plus, while the child is practicing on his/her own, you can work with someone else.
If you have kiddos who are organized or self-starters then have them get their books out and do the things they can. My child in Bigger right now sings his own hymn in the morning (and we all love hearing it fill the house).
He also does his own copy work in the morning and gets out his needed books by checking the guide.
Of course, he didn't do these things when beginning Bigger, but as he is getting further into the guide, he wants to do more on his own! All of these are time savers and pay big dividends to the parent! Plus, the child is happy to be moving along rather than waiting on mama!
If you have an older child who is finished ahead of time and is waiting on you, have him/her jump in and help teach one box from a little one's guide for 5-10 min. We do this at times with our older boys, and it helps keep things moving along (and helps me get to the older child faster).
If you give your kiddos a morning recess break all together, you can use that time to check the older's work and make sure they are staying on track. This keeps you on top of how the more independent kiddos are progressing (and saves derailment later in the day). Once the kiddos' work is checked, have them clear those books away, as clutter is a joy stealer! (At least it is for me!)
We have our boys place their books/notebooks/completed work in their own pile on our kitchen counter next to our stove. Each child has his/own space for his pile. That way, as time allows we can check their work. Then, when we have checked it, we move it to the other side of the stove (in a pile), so the kiddos can see the works been checked and put it away. Anything that has been corrected but needs attention, we don't move to the other side (as a reminder for us to go back and help the child redo that subject). For our littles in the teacher-led younger guides, we check and put away as they go (not allowing them to put anything away until its been checked).
Anyway, just some brainstorming ideas that may help or get you thinking of things that could help.
I'm sure you already have discovered many of these things, and probably more that I haven't listed.
We try to think of our family as a team, where everyone must be willing to help one another out to get our day accomplished.
If we have appointments or places to go, we will warn the kiddos the night before so they can plan accordingly and are not taken by surprise.
Often, our olders might work ahead that evening or get up earlier in the morning then, so as not to be behind. We do not, however, let our littles get up any earlier on those days, as once they're up the day is officially underway for us all.
Blessings,
Carrie
We have so many health issues and food is our greatest medicine here, along with genetically dictated supplements. I have had to work carefully with my middle ds in ways I suspect you are all too familiar with. Several things have helped, and I bet the space in your room will be a key player for your ds. Another key player for us has been GABA/glutamate balance, and SLEEP 
My time with the Lord is so important in helping everyone else to have peace, and when I don't make it prioritiy, time just slips (crashes and burns) away. God grant you peace and his spirit of wisdom and revelation as you sort things out.
and I think they would really help a lot. I think that I might not give the boys enough credit for being able to do some things on their own. I admit I've never asked them to get out some of the books, or look up what they are to copy, etc. Great idea about the Emerging Reader books. I know that would speed things up if they went over it before. The 8 yo just finished Buffalo Bill. I have the 10 yo reading Capyboppy right now. I've just been having him read about 20 min. I bet he could get more read if I had him looking it over before we read. He is one to study the pictures, read a little bit, talk about the story, etc. I don't want to squelch that because he is a thinker and I like that he's interacting with the story---but I'm sitting there thinking in my head....yikes, the time is ticking away! 
I need to let go a little and let them try--even though it won't be perfect.