As he progression in FFxiv is linear, hard hitting/hard to deal with "cuz randomness included" fights would not create such an exponential increase. It would if stats were the same than back in solo FF, when your damage/HP skyrocket after some stat tiers. In FFxiv it would give fight with gradual increase in difficulty. As opposed to the spikey difficulty increase we have now.
I'd say that current difficulty increase is big step, slow decreasing exponential, back to standard (in Coil). You get ravaged by insta kills, you learn them, they never hit anymore.
If we are being honest, when we master dives and twisters and play with 7 consistent players, T5 is relatively easy : damage spike every 30 seconds, a few adds to deal with and it's done.
Gradual increase in difficulty may not work on every fight (it would not work at all in T5 considering twintania has only 1 attack besides her mechanics), but you could save the spike for the last boss (T5, T9, T13 ?) while proposing a "close to linear" increase in difficulty. The slope can be adapted to fit the challenge too.
As for the fixed rotation, they are only an excuse to justify the overuse of instant kill mechanics. Of course randomized pattern would be disastrous in a fight where 90% of the attacks kill you on the spot. But if it was only 10% of these attacks ? You'd get a "randomisable" fight. And not necessarily easier (you acknowledged it yourself)
When the timers are made so that every skill comes out in a precise order, that's called a rotation (hence the mechanic > mechanic > mechanic > mechanic ). T5 is a bit particular because all the mechanics are timer based and overwrite themselves, it the order DK stun > plummet > DS > whatever else. In T6 (for example), I have never seen mechanics overwriting others. Nor in T2 (except phase changes, but it seems normal right ?)