OP has it mostly right. Though what really needs addressing is the fundamental fact that the combo system really adds nothing t game play as it stands. Outside combo, abilities are worthless (and no, super highly niche situations don't count), and even some weapon skills only exist as part of the combos. For example on WAR, heavy swing exists purely to lead into the other 2 combo chains, and skull sunder only exists as a mid combo before butcher's block. There's no reason for this. It also puts lots of emphasis on needing to get that final combo in as it's the one that matters; which is beyond annoying when the boss jumps/goes invincible just long enough for you to lose combo, let alone when other abilities take away your combo status (looking at you PLD casting clemency). By comparison when I used to raid in SWTOR on my Juggernaut, I could have moments where I am not on the boss, then get right back in and pick up my rotation where I left off. In XIV you usually have to just restart completely from the beginning which is just annoying.

Outside MNK who is a bit of a unique case with combos, they could be deleted and reworked and the game wouldn't suffer. I'd argue it'd be better for it. I'd rather have 6 weapon skills in my rotation to use that each had some role by themselves, outside having 6 where 2-3 of them exist purely as button/ability slot filler for the ones I want to use.