That helps so much! You are doing an awesome job with your dc, and your helping with the semi-independent boxes at the start and reading aloud the storytime is just right! I only wanted to be sure that each of your dc will be well suited for their respective guides and not require too much of you to be successful. 
It sounds like older ds is ready for RTR. Sometimes, I think you just have to make a switch mid-stride, or you'll always be finishing something out and may not get to the right placement when needed. So, as older ds is wanting his own thing, and one of the primary goals of education is for students to become self-motivated, responsible, mature learners (lifelong goals), I'd reward that drive and let him stop CTC and start RTR now, or as soon as possible, still taking whatever breaks/pauses/stops you would have done. (I have found it is not a positive to promise a break and then not take it, as dc like the ebb and flow of school and breaks as much as we moms do. ) I'd probably have your older ds read his own Storytime, though you can make that call depending on how much of you there is to go around. RTR was where my sons started reading their own Storytime, but you will know best there. I'd keep your younger ds going half-speed in CTC. Hypothetically, as ds has about half of the CTC guide left and is going half-speed, older ds starting RTR now should finish it about the same time younger ds finishes half-speed CTC. I think you would love each of your sons each having their own guides to work out of, and you can easily have a midpoint meeting time with each of them. I would let older ds know that you expect him to rise to the challenge of the RTR guide, working carefully and following directions in order for independent boxes to the best of his ability. Let him know you are rewarding his drive for wanting to do more and that you can see he is becoming a young man and needing the opportunity to take on more independence and maturity. Half-speed CTC and full-speed RTR with 2 dc having their own guides won't be that hard to do, especially as you've indicated that dc would do well separated in your opinion at this point. What are your thoughts though? In Christ,
Julie