Rankings are entirely subjective and should be scrapped altogether.
Similar to what TBGKon said:
If you want a playoff that includes close to every FBS conference (because they are all theoretically at the same level), take 8 of the conference champions, and seed them similarly to the pros, with teams with an equal record being ranked based on things like point differential, yards for vs yards against, etc.
Reduce the regular season (in normal years) to 11 games if you have to, because I know these are (supposedly) student-athletes and they don't want them to play too many games.
I know neutral site games are common anyway. Schedule 5 home games, 5 away, and 1 neutral for as many teams as possible, and outright BAN schools in the Power 5 from loading up on cupcakes. It's very simple, very fair, and it's what literally every other level of college and pro sports in North America does.
I realize that in Canada, University Sports football only has 4 conferences (also because in Canada, we don't consider colleges and universities to be the same type of learning institution), and so it is FAR simpler to stage a conference-champs-only playoff, but something has to give.
You can even keep the bowl games, including the New Year's Six. I have no issue with rewarding teams that have a good or even OK season with a pseudo-meaningful game at the end of the year. Just stack them. Four serve as national quarter finals, two are semifinals, as now, and then have the national championship. Done.