simple way to reduce button bloat without changing gameplay

[Zebraoracle]

You're right, just merging combos wouldn't work for something like BLM which is why I don't think it's the solution either. But I was personally never too fond of combos anyway because unless they are branching combos that give you a meaningful choice, they are extremely wasteful with hotbar slots and don't add anything to the gameplay. Pressing glowing buttons in order for slightly increased potency isn't the pinnacle of skill expression, no matter what some people claim. And your entire ARR is basically just "press 123 in order" with things slowly getting added during HW but you feel the extreme low level skill pruning in lv 70 Ultimates.
If the only engagement comes from "Well, you could accidentally press the wrong button in panic!" then that's really not much compared to, say, miscalculating mana ticks and being left with a short AF phase when that wasn't your intention or starting your melee combo at the wrong time.

Branching combos are good though and they're not wasteful either. And they offer a simplified but solid version of your rotation if you're just not at a point where you're good at doing them correctly. I was one of the people that just used 123456 on MNK when levelling it in ShB. Was it ideal? Nope. But it did the job well enough and I wasn't neck-deep into MNK gameplay.

Of course I'd personally prefer to have a good revamp with a close look at a lot of skills, the 2min burst meta that can go to hell and adjust it all but I know it won't happen.
FFXIV relies a lot on having almost 3 hotbars full of fluff and the sheer amount makes it seem complex but then about 1/4 of it is for aoe alone, 1/4 is "fluff/ niche/ you never use it lol", 1/4 is "flat potency press on cooldown" and the remaining couple of buttons are your core rotation - which then boils down to pressing 123 and something else in the correct order.
And I've seen in other games how you can engage a pretty engaging gameplay with just 5 buttons. And yes, that would work in FFXIV as well.
But quantity gets mistaken for quality here on many classes and the below current expansion gameplay gets increasingly worse.

I will always tout Guild Wars 1 (definitely NOT 2) as having the best combat in an online game like this. You come up with your own build, you slot eight skills that work together, it's not based on a rotation... Gosh, if FF14 and GW1 could be combined, I think we'd have the perfect game tbh, at least combat-wise.

[TheDustyOne]

Black Mage could also consolidate Fire IV and Blizzard IV into one button based on your Astral/Umbral status, same thing with Flare and Freeze. Alongside the aforementioned Leylines/Between the Lines merge that saves 3 buttons without removing any skills.

While I would also prefer to merge abilities that have the same function but are merely just single target/AoE variations, I still think there's some value in having AoE rotations themselves be separate and simplified versions of the single target rotations. A quick example off the top of my head would be Warriors AoE combo being both a generous beast gauge generator and their buff refresh rather than having 2 combo enders in their AoE rotation. Considering that it's primarily casual content that makes most use of AoE abilities, I think it's fine for those rotations to be much easier. I think it's only abilities that serve the purpose of being the 'AoE' version of another ability that should be merged (Upheaval and Orogeny, Foul and Xenoglossy, Senei and Guren, etc.), while AoE abilities that merge a jobs mechanics together should be left alone (Most AoE combo enders fit here really, typically applying a buff and generating a fair amount of resources at the same time).

Monk is the only job I can think of where combo consolidation wouldn't work without a serious rework to Perfect Balance and Form Shift. I would've said Samurai as well back in Shadowbringers, but since the buffs from within their combos now also apply with their combo enders under Meikyo Shisui, combo consolidation there wouldn't be an issue; 3 buttons to represent the Gekko, Kasha, and Yukikaze combos and Meikyo would change those 3 buttons to their enders.