I did find a very nice and very expensive current sense amplifier designed for almost exactly this purpose.
But it's not quiiiiite drop in. After staring at the lm3409 sense circuit I confirmed that yes, it needs a Vsns- to be at least 1.25V above ground. That can probably be done by referencing the amp ground to the MCU Vcc for instance, but let's come back to it after finishing up the baseline design.
So, thinking about iadj a little. A couple of things. Right now, the iadj voltage is 4.2 V times the iadj PWM duty cycle. Except I think that 4.2V is actual battery voltage? You are using a single battery Vcc design here right? This is unfortunate. It's going to make output fall off as voltage sags, just as it would in a direct driver!... because the battery is directly driving the iadj pin. That's pretty obnoxious for a buck driver. So you might want to bring back out the ldo.
Edit: I got confused on PWM RC optimization. I'll come back to it.
Still.. an RC of 1 would have a nice touch that all mode switches will be soft.
Another point though, right now full scale is at a duty factor of about 25%. If we use a voltage divider we could make 100% be full scale. But is 50% still 50%? I have to scratch my head on that.
Finally, did you actually hook up uvlo? Do we want voltage protection there or in the attiny? Having it redundant seems unnecessary and could cause confusion. Plugging Vin straight to uvlo will just override it, and you get rid of two resistors too.