mommyofmany wrote: We started off with The Reading Lesson and the Bob Books and all of the sudden she started reading everything she could get her little hands on. She is just starting to write some letters and numbers and isn't as advanced in her writing skills as she is with reading at this point.

I think MomtoJGJE gave you some good insight on being able to start LHFHG at 4.5.
I wanted to share coming more from the place of having an oldest who is very advanced in reading (far above grade level), and who figured it out on his own very young (started reading at ages 3 - 4). His writing has never kept up with his reading. So what I want to share is that at age 10 Preparing is still a good fit for him for 4th grade due to his writing skills. In other words, you don't know what will happen down the road with her writing or whether she will or will not be able to combine in the future.
I thought too I would share my daily verse with you that I adopted when my own child with Down Sydrome was born because ever since then it has been too easy for me to 'worry' about her future (health or otherwise) or get overwhelmed with daily life:
Matthew 6:34 (NKJV)
Jesus said, "Therefore do not worry about tomorrow, for tomorrow will worry about its own things. Sufficient for the day is its own trouble."
I know it isn't easy, especially with the challenges of a DS child in the home, to always cast our cares upon the LORD. The LORD though has been teaching me to commit & cast all my cares, worries, troubles, future concerns, and burdens upon Him. He has also been teaching me in home schooling to make decisions based upon our needs today, and not to worry or think about what it will look like in the future until we get there. Perhaps when you get to the place of needing 3 guides you will be ready!
It sounds like starting her now in LHFHG half speed is a good plan. You could start and see where it goes.
Your home schooling with a T21 child has inspired me in the past & is inspiring to me today. Thank you & God Bless.
HTH