Its length doesn't bother me. Its length compared to UI's length does. That combined with AF/UI timers being completely worthless now but still existing makes the entire situation a farce. Frankly, if they removed the timers and dragged the class back to HW's 4 fire 4s per rotation and gave the class a real umbral ice rotation, it would feel better. None of this addresses how BLM is also still functionally an over-time class in a burst damage meta, but at least fixes core structural issues that have existed since ARR, but only became a real problem starting with Stormblood.
I think it's fine explicitly, but there's some hidden problems with how it's handled. As mentioned prior, the damage over time nature of BLM means flare star doesn't do real damage compared to anything else BLM already does, including despairs and even fire 4s. It is an increase, but only in the way a normal midare setsugeka is an increase over the basic rotation. Only the numbers aren't as big as even PCT's giant doom meteor so you kind of get the original midare with the new midare's potency but no auto crit/dhit. This is more a balance and structure issue than anything, the ability is fine in animation and intent, but its place could use some looking at.
It's possible to not clip because even going high thunder->fire 3 is still ~2.5s of wiggle room. The problem is, because of timers and, as you say, the 30s buff duration, I often find I can't cast it and it, simply, falls off before I have a chance to recast it, with the buff falling off sometimes seconds later. Thunder as a whole needs a complete redesign. It was functionally carried by the absolutely ridiculous damage it did with a proc enabling it even as an emergency movement tool, but always felt bad from HW on, confused new BLMs and made an interesting discussion between safety and damage from SB+, but overall just isn't a good mechanic. In a hypothetical rework, Thunder should be a stackable buff, rage bar, or some other mechanic that enables ad hoc movement. Combined with removing AF/UI timers, it would be an interesting replacement to enable more movement options.
As it exists now, Thunder is basically a better Scathe. If you clip the DoT it's decent damage. If you don't, it's about the same damage over 30s as ye olde thunder from SB+, but it's a textbook orphaned mechanic. It's there because of tradition and little else. I'd prefer thunder were kept, but reworked, because I'd like BLM to be a BLM, not a fire mage that occasionally tells the enemy to chill out.
This can be completely solved by simply removing AF/UI timers. Also, your last statement about relying on server tics is interesting, but I've never run into a situation where even B4->transpose caused me to not get the full benefit. However, as brought up prior, in addition to removing the AF/UI timers, trimming back the AF rotation and building up the UI rotation would do wonders towards making this feel better. Black Mages are known to be masters of all destructive elements, not just fire. It would be nice to bring that back in.
I could not care less about this as-is. The class has plenty of movement, especially given Paradoxes auto proc into firestarter, meaning you have 2 instants for every 8 manual casts. That combined with 40s swiftcast and triplecast's 60s cooldown, nevermind xenoglossy's 5 casts per 2m provided you don't accidentally overcap it (why did so many classes get overcap protection but not BLM, or even GNB for that matter?) In terms of raw mobility, BLM is remarkably similar to PCT, with the specific exception of having a much harder time using oGCDs without clipping for significant chunks of the rotation.
Compared to standard EW, the class is substantially more flexible on movement now. There's a reason I was screaming from the rooftops throughout all of EW at how bad BLM actually was, but the crowd of players completely addicted to nonstandard rotations drowned out the rather serious problem of BLM's core design. As it turns out, fixing nonstandard revealed just how bad BLM actually was. DT is an objective increase over EW's standard rotation, but holy hell is it still in a really bad place. Fun, but bad.
That all said, I'm kind of expecting BLM's main GCDs to get PCT treatment. 1.8s casts with 2.8s recasts for things like fire 4, or 2s casts with 3s recast despairs/flare stars. The class, now revealed to be in the dire straits it was in throughout all of EW even to the nonstandard junkies, is going to force the devs to give it a rework. Possibly even a rushed rework. Its problems are more than just potency related. It has no burst, it clips oGCDs too much, the class is almost entirely an AF phase now. Now that nonstandard is no longer hiding its issues, the class can finally get the rework it has needed since shadowbringers, but was always overshadowed by how much worse other classes were. Very much monk treatment.