I'm no BLM pro but from what I read from you I gather that you might be misunderstanding the point of instant casts entirely in BLM's (and perhaps all casters') design.

Now that the stances are 15 seconds long and more comfy, there is no real way you can drop them by refreshing your Thunder DoT as long as you know that you can safely cast Fire 1 (or 3 with a proc) to refresh Astral Fire before it ends. Thundercloud offers some flexibility on when you want to refresh your DoT and offers a GCD where you can move or weave, that's it. It might seem to be a waste compared to a cast of Fire IV, but DoT uptime does compensate for when you cannot DPS much or at all (when moving for a mechanic for instance, and even then you have abilities like triple cast and Swiftcast to avoid "spamming umbral soul", which you should not do in an encounter unless the boss is not targetable)
Instant spells are weaving opportunities and movement tools, and arguably Xenoglossy is a burst opportunity with some flexibility to it. If you lose your AF or UI because of instant casts, you might be doing something wrong with the overall rotation of BLM.

I think BLM is currently at its highest, design wise with Shadowbringers? From what I read and my current experience of it at least. I don't think it's a Time-based job, but rather a job where optimization lies in planning ahead any mechanic / movement. There's nothing in its design preventing BLM to perform, and it actually has efficient tools to do so.