You'll need to do your promotion/relegation manually using the wizard. The game will want to promote your top three clubs by regular season finish, so if the 4th - 6th place finisher wins your playoff, you'll need to make sure to adjust for that manually. Also, because your top two teams aren't part of the playoff, whoever wins your playoff tourney for that tier will be listed as the overall champion in your league history. See pictures for reference.
What you want to do is definetly doable, just requires some manual intervention.
Check out Lucky_Will's thread -
https://forums.ootpdevelopments.com/...d.php?t=342456
He has an intense setup that he's been sharing in the fictional simulation forum with tons of teams, tournaments, etc... he's definitely someone I'd look to for knowledge on P/R setups. I'd bet there is a way to solve for the league history issue I described using the tournament functionality in the game... I just don't know how to do that.