Conference play should be based on a full round robin. Any “conference” that has split into two divisions, to my mind, is no longer really a conference. The divisions are the real conferences, and the larger entity is just a television marketing gimmick.
Eight teams is the right number for a playoff, but not because of academic or injury considerations. I’m old enough to remember when those were the reasons given by the colleges for opposing a playoff back in the ancient days of a ten game season. I took it seriously then. there is no credibility to it now.
That said, any credible playoff needs to include all major conference champions to avoid endless strength of schedule arguments. It needs to include all unbeaten teams. And it needs to have room for the occasional really outstanding team from a minor conference; otherwise the lower half or more of D1 will be shut out in perpetuity and might as well just leave. On the other hand, 16 teams is too many. There would be too many two and even three loss teams, and second and third place teams from the major conferences.
I think it is a matter of time until the big football powers split off from the minor teams anyway. The big boys make too much money to let the little guys write the rules. Furthermore, they can rid themselves of the NCAA.