two things. I set mine up tight. I use hard straps...windsurf kine.
This years naish board have actually done something that was missing before..they have build up winder the toes. It takes off a lot of the pressure on your ankle.
Heel should be off the board..lean out more and you should be fine
I found the deck too flat, both for waterstarting and riding.
I think the latest boards have more ergonomic rail shapes - like windsurf boards.
I cut a pair of dakine pads in half and cut a wedge-shaped piece of foam out, then stuck the top surface back down so the pad was sloped. Its hard to explain it exactly but its done so that the pad is comfortable with your heel on the deck. I just want the board for light wind across the wind using three 25cm fins.
I had to use twintip straps because of the extra height of the pad and windsurf straps were not big enough.
I haven't used the board with the pads yet but the pads and straps are a comfortable fit.
Just thought I'd post a photo since you mention the flat deck being a strain on the ankles.
When windsurfing, I set the straps to fit just at the top of the slope on the top of my foot and sometimes have to wiggle the foot slack in preparation for taking it out if I've been pushing hard with a lot of power as the foot then slides a little inside the boot and has gone a bit further in.