I'm not sure it's possible to do this in such a way that doesn't fundamentally alter the fight.

So let's say you have your checkpoints at phase changes (the most logical place for them). On your T9 example, you'd checkpoint after the Heavensfall phase, right?

So let's say you wipe to somebody pooping thunderstruck in the raid. Horrible failure, everybody dies. The fight restarts... but now it's starting at your checkpoint. Stop right here. What does this look like?

Are you restored to full hp/mp? Cooldowns reset? What if party members died to Heavensfall and weren't revived before elements? What happens to them?

The issue with trying to do something like this is that every decision you make in a fight has to take into account what happened before it. So if somebody makes a mistake earlier in the fight, decisions were made based on that that alter how you approach the fight going forward from that point. Without that, what you're really suggesting is to turn one long boss fight into many tiny pieces of a boss fight. I don't think this can work the way I think you're describing it.