I am doing Bigger and Preparing this year and this is definitely my heaviest teaching year. However, I also work from home and that is what seriously add to my craziness. HOD is the only reason I actually get school done.
For me, if plan to accomplish a day in a day, then I really need to stick to a fairly rigid schedule. It is doable but you must be fairly steadfast to stick it out. Also, my son still needs quite a bit of handholding. He struggles with reading a bit still. So usually I listen to him read all of the directions out loud to make sure he understood what he is doing. He is getting better but he still needs me. And for some of the history and science books, I am making him read them out loud as well. He is actually a VERY hard worker and likes to finish one days worth of plans in a day so I try to let him. But because he reads some stuff out loud to me, he does take a bit more time on some days. That is okay by me though
What helps to make up some of my lost time on him is that my youngest reads all of her history and science to herself. And depending on the storytime book, she also reads that alone as well if she wants too. My youngest is VERY independent and my oldest is as well. My oldest hardly takes up any of my time. She reads all the "I" boxes to herself along with the storytime books as well this year. I only spend about 1 hour a day with her. With my youngest, I do still spend about 1-1.5 hours with her. But with my son, depending on the day, I might have to spend up to 3 hours with him.
Yikes! For him, his reading is coming along well but he also highly struggles with spelling making independent write minimal at best. Three out of the four days in the guide have writing activities. We have a good rhythm on how I help him but some days take longer than others with my help too. For example, on creative writing days, I may or may not originally write what he says and have him then copy it. On science question days, he writes it down all by himself using me as the human dictionary sitting next too him. But on written narration days, I always write down what he says first and then he copies it. Along with that, on written narration days, I am still helping him to create and encourage more vivid descriptions in his writing. So, it just varies how much I help him. The good news is that once you get to Preparing, your guide is written to 4 days a week instead of 5. And some boxes are writing to the student. Sometimes we do need the 5th day to finish up. With the ages of your kids, if you plan to just do 4 days a week with your youngest and 5 days with your oldest, you can probably create some space.
All in all though, even though this is the most teacher intensive year, it doesn't last forever. I love having my kids in their own guides. I love having them go at their own pace. I love having them do things alone. For me, this is worth it. While I am spending a lot more time with my son this year and helping him along, that is also worth it to me. I am investing in training him on how to do these tasks because eventually he will be doing them alone. I want him to be able to do them well. And finally, we started HOD 2 full years ago. The first year was definitely the most challenging. That was the year my oldest started in Preparing. My younger two actually started in Beyond (I split them last year). I found that year was hardest on my oldest because there were so many areas she didn't quite understand with HOD. It really wore on us. This year, even though my son needs help reading what the text states sometimes, I don't feel like this year is nearly as hard as it was the first year for my oldest. Ironically enough, my oldest also really struggled with reading and spelling until she was 9. At that point, it was like a switch flipped. She is amazing now. But for my son, the switch just hasn't flipped yet. Once it does though, he will be more independent. But until that day, I will just helping him as much as he needs and go at his pace. (And ironically enough, my switch didn't flip until 5th grade either! Now I am a voracious reader but this was definitely an area I struggled with significantly as a child).
I know you mentioned your son struggles with writing and not reading. My hope for you is that one day, his switch will flip too. The great thing with Beyond I found is that the kids start out by copying poems. My son could hardly write at the beginning of Beyond. We set a timer for 2 minutes and gradually increased it through out the year. When we started Bigger, it was a bit of an increase in writing but he was fine! And by the end of Bigger, he was writing a ton! Now in Preparing, there is a bit more writing than in Bigger but he has not once complained. I love that with HOD it is more copywork than anything in the younger levels. I found that because my two oldest kids can't spelling well, this was the best thing for them anyway. It helped them with reading too because they had to copy things they read. But for your son, I do think that if you do Beyond, you will probably have no problem getting through. And then once you start Bigger, you might even able to have your son take on some of the reading. By the time you start Preparing, since your son reads well already, I don't think your day will take nearly as long as mine. I do actually think your kids will be fine back to back as long as your other two kids can work fairly independently. I know that has been my saving grace this year! And for me, my son has finished a level each year so far but if he doesn't finish his level this year, it is totally fine! We will just pick it up where we left off last year. That is what I did with my oldest last year. She didn't finish her level for CTC because she has something else she worked on for about 2 months with AWANA. So we went half speed in CtC to give her a bit of extra time. It was actually wonderful starting school with a child who was already well acquainted with the guide. My thoughts are that I wouldn't mind having all my kids on this type of schedule. Sounds like you might already be doing something like this anyway if you plan to go a little slower with your youngest.
I know I probably didn't answer much of your question but wrote what I felt like the Lord was leading me to say. Hope it helps in some way.