OP, remember how doubly blessed you are to have 2 wonderful children capable of attending higher education.
Additionally, you are very fortunate to be in a financial position such that you describe - being able to afford to be full pay at their choice of undergrad schools where they get accepted. This puts you in a significantly higher financial status than the vast vast majority of families of college students, since most families struggle to afford each year of undergrad.
I was initially confused by how you referred to your children as D17 and D20 - I thought at first these were their ages but it sounds like you are referring to their HS grad years. In my point of view, from a family that has benefitted greatly from 100% needs-based aid at elite schools, it is entirely appropriate to do your best to meet your children’s different needs because the costs will inevitably be different, as long as they are in different years, and attend different schools.
It is likely that whatever you decide, you will be discussing this with both your children and that they will both know that you are doing your best to do right by both of them. You aren’t playing favorites, you are not keeping a secret ledger where their ultimate inheritance is based what was spent on each of them since birth down to every penny, because they are different kids with different needs. You are trying to be fair to them and while that doesn’t necessarily mean equal, your goal has always been to meet their needs.
Now you understand how everything works (better than you did before you made you initial agreements with your D17 before she made her decisions). It makes perfect sense to me to ask your older D now for her thoughts on how she would have reacted if she had the same option then - and with 20/20 hindsight you believe that since she landed at a great fit school for her, it probably would not have made a difference to her, but you don’t want to change the rules for your D20 without your older D’s input. I am sure both of your kids are grateful for all the support, they know you are not trying to use your parental money to control their lives. It’s just that you know families of all kinds of financial situations do their best to meet their kids’ needs and to set them up to be successful grown-ups - but you don’t want there to be resentment so you are talking to them as adults.
Since there is not an unlimited well of money flowing, you are also attempting to teach your kids another valuable lesson in life - not just that education is valuable and worth paying for, but that there will be other needs in the future that are also worth pursuing.