I've had a pair of the Softstar Runamocs in their widest size, and it was definitely much much wider than the Altra Instinct toebox. I've got the Altra Instinct/Samson models.
Though keep in mind that the Softstars won't have the same cushioning as the Altra Instinct or Lone Peak. The Altras have, I think, a 20mm stack height while the Softstars with the Trail Sole and footbed might have a stack height of 8mm or so.
Transition time can take a while. For me, I spent over six months transitioning to full time minimalist running. Even now, I get the occasional soreness in the Achilles and the warning of pending Top Of The Foot Pain, especially when I've done too many half marathons back to back. So for a couple of months out of the year, I give my feet a break and run in the Altra Instinct. Perhaps, plan every few weeks to give your feet a pre-planned "minimalist sabbatical" and let them heal, then get back into again.
There's a high/low heart rate limit option for the Ambit. I've never activated it, but it can be found in the advanced settings for the custom profiles. That will really only cover one zone and tell you if you're inside or outside of it. Though now that Suunto has upgraded the Apps portion of Movescount, there might be an App now for zone training.
Yeah, be mindful of the distance that you run/walk in them. Just think low and slow: low mileage and slow transition to going full-time in minimalist shoes. For running, I only ran in them one day a week for the first two months, and those running days were my short distance days.