Before my three kids started college, I tried to be legalistic and write up essentially a contract that would outline what I would pay for and under what circumstances.
It didn't work. Circumstances arose, plans changed, unforeseen gray areas were identified.
What I would recommend:
1. Have talks early and often. Communicate what you can and can't, will and won't do. This way the kid knows what to expect and can better navigate high school and college.
2. Talk in general principles, not rules. Instead of reimbursing B's at 85% (rule), explain that you'll only support them as long as they're making good progress (principle). This means you can handle gray areas as they arise rather than trying to pre-legislate them out of existence. It also means more communication and understanding.
In general, supporting them along the way as long as things are going well (and cutting off or dialing back support if they go astray) will be less expensive and taxed better than any of the carrot/stick ideas in the OP. Loans, for example, have origination fees and sometimes interest payments. Paying along the way could be done from a 529 rather than paying off loans from a taxable account (with possible realized cap gains).
I'll add that one of my kids didn't really respond to the cliff/threat/rules thing very well. It stressed him out, and with that plus COVID he had a really bad semester. Your kids may respond well to it, but they also might do worse rather than better.
As for skin in the game, I told my three kids that there is a pile of college money for their basics as long as they were getting a marketable degree with a plan to use it to support themselves at graduation. Anything left over will be split among them, and in inverse proportion to how much of the pile they spent on their education.
One of mine is taking a pricy route, the other two were/are on the frugal plan. Pricy kid will get a small chunk at graduation, frugal kids got/will get enough for a decent car.