Amy,
Take a deep breath and give yourself some grace here - this is going to take some time to get into a good routine.

Schooling from 8 AM to 1 PM or so isn't bad for starting 3 guides, and multi-tasking takes a bit to get into a groove with.

From reading your post, it sounds like everyone is needing you all at the same time, and when that happens at our house, my head is spinning, and I am tense and terse too.

There are some good tricks to cut down on this everyone needing you at once.
First, I think I'd do Bigger Hearts singing of hymn with everyone before breakfast. At breakfast, I'd do Bigger Heart's poetry with all. Each of these should take about 5 minutes.
After breakfast... 20-30 minutes block:
LHTH child with you
middles playing quietly independently away from you, consequence for interrupting or being too loud
BHFHG child doing totally independent work of cursive, poetry copywork, and studying her dictation
Next 45 minutes block:
LHTH watching an educational video.
middles still playing or joining LHTH child and watching educational video
BHFHG with you, suggested order of boxes: Dictation, R & S English (have her only write 3-4 things, rest done orally), Bible, Reading about History, rotating box finishing it out on her own
Next 45 minutes block:
LHTH having snack and doing something at table by BHFHG child like puzzles, coloring, playdough
middles with you, left side of plans
BHFHG child finishing rotating box and having snack with LHTH child, then helping LHTH child play her things, this is a playtime for the LHTH and BHFHG child, must be quiet, must be led by BHFHG child, no interruptions for you and middles
Next 45 minutes block:
5 yo dc play with LHTH child, choose something they can play well independently
6 yo and BHFHG child with you, you read BHFHG's Storytime box to 2 of them and do follow-up questions
Then, 6 yo reads ERS to practice independently. BHFHG child is with you, suggested order of boxes: DITHOR, math, science, she can finish science out on her own. Whenever dd is busy independently within her DITHOR/math work, have 6 yo read ERS to you and ask the ERS questions.
Next 45 minute block:
6 yo plays with LHTH child
you are with 5 yo dc doing phonics; whenever BHFHG dd finishes her work, she is responsible for playing something with LHTH child away from you because BHFHG child is done
5 and 6 yo dc come back together to finish out right side of LHFHG plans
with you
This would have you finishing school in 3 1/2 hours. You can figure another 1/2 hour for transitions, so if you started at 8:00 you would finish around noon. I would be sure not to add anything to the day, draw out discussions or activities, or allow dawdling. Clip along with each thing and move on, and dc will adopt that pace too.

These are all just suggestions, and meant not to be the ultimate plan for you, but just meant to give a picture of how multi-tasking can work. The most important thing will be to make sure your middles can play without interrupting, as well as your oldest play with your LHTH child without interrupting. This will balance your time among your dc.

I would not have your dd who is 8 yo doing extensions for BHFHG, as they are intended for 10 and 11 yo. You can put the extension books in a basket in her room, and let her read them at her leisure, but I wouldn't require her to read them. Bigger Hearts is a full enough program for an 8 yo without them.

What do you think? Is this feasible? Is there something else that I could help kick around some ideas with? I do think that you just started and this will take some time and you have a large family - but I also think that you can do this if you just divide your time and multitask.
In Christ,
Julie