I think it's a great idea, and a great RP-friendly use for Insight. Assessing the "opponent's strength" seems just like what Insight is for. And I don't think it would be too OP to allow it every time - it's not like it gives you a bonus to the roll, or is an automatic win of any sort.
As a side note - I love that they're thinking about flat checks. While I don't really have a problem with dice in a D&D game, I'm not a fan of RNG in general. Even for one person, having different ways of making the check is great: I could see myself using dice for the first playthrough ("D&D authentic experience") and then mostly going with flat checks for subsequent playthroughs, while reserving die checks for luck/wild magic themed PCs.
