I've never heard of cellulose batts, just blow-in. Every batt I've ever seen has been fiberglass. I had some electrical that I wanted to clean up in my attic, but once I blew in insulation, I can't see it anymore so I don't think about it.

The solar power attic vent is what you want, but I didn't suggest it because I thought the roof had already been replaced. Adding soffit vents to work with the powered attic vent would be ideal, but I understand the issue of the narrow soffits and Hardie Board being more difficult and less desirable to cut a bunch of holes in. As an alternative to a heat register, strips of pegboard can be pieced in between rafters, then painted to match. That's probably what I'll do on my own house. I'm envious of you having an empty attic to work with.