The timer having a failstate is part of what makes it so fun. Mastering that 15 seconds, and fully planning out everything you need to do so you're not stuck in a situation where you've got four Fire 4s left to cast, you've got three seconds to use paradox, your DoT is about to fall off, and you're about to overcap on Xenoglossy. If you remove that aspect, you remove the fun of Black Mage, and make it into just another ranged DPS that stands still to do damage. it turns optimizations from something you're always keeping in mind to just a matter of keeping the GCD rolling.
After clearing the first extreme trial, It almost seems a bit too lenient, and restrictive at the same time. I barely ever use Triplecast because I've almost always got a movement button available. We get Paradox and Fire 3 to use whenever we need to just as long as we don't use both of them before our third Fire 4. Thunder however heavily punishes long fire phases where you use one too many xenoglossys. Dropping Fire phase in Endwalker was a simple matter of just using Fire 3, and blasting your remaining Fire 4s before Despair. Now it's a much more substantial DPS hit than it already was because of the lost Flare Star. Before, I would always keep a close eye on that timer so I made absolute sure to get three or four Fire 4s in before Paradox, then the remaining before Despair. None of the main rotation were movement tools aside from phase changes, and Ice Paradox. This means we've got to plan very closely what we use each charge of triplecast, swiftcast, xenoglossy, and even instant Thunder procs for. Now I don't feel the pressure to even bother when I know that I've essentially just got two free (reset timer) buttons to use. Also, this might be my own personal thing, but I'm finding myself gambling on timers much more now. I'll regularly try to start a cast when the timer says 3 in hopes it finishes just in time to cast Paradox at the end, and be fine. That wasn't the case in Endwalker because of how strict the paradox cast timing was. It also imbalances your fire phase to have up to five GCDs before Paradox, and three after.
It might seem like I'm just complaining that it's easier, and maybe it is, but I also thought it was better designed before, and encouraged the right kind of deliberate planning and executing gameplay.