Thanks for the quick response! That sounds like it will work.
In my case, I'll have 400W total with two dual-zone extuders, with the bed being handled offline, but if I understand you correctly, all of the heaters are metered at their connection to negative. I had initially expected to use SSRs, but I was thrilled to learn that the extruders heaters can take 5A.
It's like you've anticipated everything!