No strong feelings any which way, but here's what I did.
I have three kids: DS26, DSalmost21, and DD19. Each had various combinations of UTMA, ESA, and 529s, but most of the money was and is in the 529s. I used my state program and chose the investment option. There are also age-based options that work like target date retirement funds but for college. We don't have a guaranteed option like you're talking about; we do have money market / stable investment funds, though.
I treat the college funds collectively and freely shift money between the accounts as needed to reflect changes in the kids' needs. I also realized that by treating the accounts collectively, I could effectively rebalance by shifting assets around between accounts (which I can do any time I want on my state 529 website) rather than within an account (which I can do three times a year I think).
I basically have projections for each of the kids college costs in a spreadsheet. I keep the next four years in bonds, and anything beyond that in stocks. Any leftovers I have allocated according to the kids' investment preferences (since it'll be their money at that point).
Whether I have had "enough" in the college fund has varied. I didn't really know for a while, so I just picked a target number, saved that amount up, and then decided if I was short I'd figure it out. When my middle son got a good financial package to his school, I was relieved and thought I had things about right. When my youngest got a full ride, I thought I had too much. Now that my youngest is thinking about transferring to a more expensive school, I don't know but might not have enough.
Really, I'm aiming for a target of having a bit left when the last one graduates. But in practice, I'll either have too much, in which case I'll hand off the extra to them, or I won't have enough, in which case I'll exhaust the college funds then supplement from my personal funds.
The oldest has graduated, and the other two are basically sophomores. As they progress and things change, I just update my plans, monitor where I am, and act accordingly. I can't do any better, really.
My kids have all gone to standard US universities, so I've had no problems using the funds for their schooling. I usually disburse from the 529 to me and then write a check to the school. I think I can also have them send the check from the 529 to the school directly.
I assumed 6% inflation in my models and am fairly certain I've done better than that in their accounts, although I don't really monitor the performance that closely. College inflation over the past five or six years has actually run below that 6% number I think. And I would say that where the kid ends up going, scholarships, gap years, changing degrees, etc. can swamp any effort at an accurate expense estimate.