Navigational error is happily not (perhaps not yet) one I can add to my extensive list of hiking mistakes/disasters.
Being of a certain vintage (in common with many here) I learned the map & compass method, and still find it invaluable. The phone satnav with downloaded OS 1:25000 mapping is a delight to use, of course, and much easier - it feels like cheating!
Hiking terrain is as varied as the skills and abilities of those hiking it, and like so much with this pastime it's an individual choice and one size doesn't suit all. Having got those mandatory platitudes out of the way though, I would argue strongly that when in potentially dangerous wild hill country it is irresponsible to go without a manual map & compass of some type, at least sufficient to navigate out to safety. Plus knowledge of how to use them.
In my case, this usually consists of a basic magnetic compass and an A4 sheet or two of appropriate mapping in plastic sleeve (25g total). It's not designed to enable my hike, rather to facilitate my escape should my electronics fail.
The most important stuff is that which we carry in our heads - the understanding of our terrain, how it relates to what we can physically do, and how to make effective use of our equipment. The thing about the map & compass method is that it forces its user to engage with their landscape, including the landscape they cannot see, in a way that the satnavs don't (at least not nearly as much). Thus, I believe that paradoxically those less proficient with map & compass navigation have more need of it than their more experienced counterparts.
The FAK requirements are, as
@cathyjc observes, highly individual - at least beyond a few common 'must-haves'. My point is that the 'generic' FAKs are seldom going to represent either the best or the lightest option, given their requirement to be a general one-size-fits-all product. You're going to end up with some irrelevant stuff and possibly lacking something relevant. But the OP's onto this. Mine's 125g or thereabouts, and being constantly revised. It also contains some stuff not really countable as medical, like water purification tablets and equipment repair patches. When you've been doing it for some time, it's an interesting exercise to see which items you're constantly replenishing and which are unused.