My 2 cents, for whatever it's worth is, you don't determine Sexton's value based on whatever our salary cap implications are, that's a fool's path. You want to keep talent, whatever that costs, and then figure out the rest. Sexton, regardless of position/starting etc... is a top 5 player on our team, maybe even top 3 this year.
But I do think you consider how high you can go and still have a tradable contract. Is paying Sexton closer to 17/18 mill a season harder to trade than 15 million? I don't think it ends up being any more difficult, probably could even go up to 20 million a year, and he's still tradable and seen as a "promising talent" etc... (aka, he doesn't hurt a trade). Yeah, 25 million a year, and you'll struggle to find a partner (but at 3 years, might not even be that difficult).
With all that in mind, I see no reason for Sexton to sign yet, if he can squeeze go from 3 years 40 million to 3 years 50 million (or even 3 years at 41 million), that is easily worth the wait, and for the Cavs, probably a very good deal still that's easy to trade if necessary.