My oldest turned 9yo this past June and in 4th grade. She reads well above her grade level, has great comprehension and is very mature. BUT her writing is behind. She has always struggled with fine motor skills. She is just now learning cursive and doing very well. I am so glad I waited to introduce her to it this year and let her have one more year practicing her printing to get it looking good.
All that to say, my daughter is LOVING Bigger! We are only one week in, but it has gone very smoothly. I have her at her own math and grammer levels. DTHR is already at her own level. I do need to keep her a nice supply of extra reading or she will want to jump ahead in our school books.

She would read all day long if I let her.
Writing is such a huge part of this. For us, I would much rather she enjoy her work and be confident in what she is doing. If my daughter had to pour 90% of her thoughts into physically making the letters she would lose all that learning about what she is writing. Does that make sense?
It looks to me like Bigger has a lot of key skills that are gently brought into play throughout the year - to get them ready for Preparing.
Plus there is absolutely no hurry. What is it going to hurt to have our children in Bigger and do Preparing next year? If all the above mentioned stuff is customized to their level then the history and such will be fun and exciting stories to them! It's not a grade level. It's a building block that we will continue to build upon in the years to come. If we are missing the foundation then they will not have solid footing.
Oh and fwiw, I had originally planned to have my 7yo son in Beyond but I second guessed myself and placed him with his 5yo sister in LHFHG at the last possible minute. I am so thankful we made the switch. I am still tweaking a few things but he is right where he needs to be!
I hope you find this helpful. I know it's hard trying to figure it all out. (((hugs)))