the benefit of DTR would be spread over a longer period of time, further adding to the issue of truncated damage.
This is false and completely irrelevant.

You're basically saying DoTs are bad because they deal damage over a long period of time.

I guess I have a hard time believing that truncated damage can 'carry over' to the next DoT tick, I think the damage calculations would just be more simple than that.
FF14 seems to have obnoxiously overcomplicated equations in certain ways. That aside, the concept of the DoT "remainders" carrying over is very minor and not a huge impact. It's the least of the 4 assumptions re: DoT tick truncation; there's no reason to get hung up on it.

DTR damage adds being spread over the course of the whole DoT would also explain why it 'seems' that it takes a good deal of DTR in order to increase DoT ticks by +1 damage,
The point is that this is no different than Int.

I could say that Int sucks for Summoners because it takes a lot of Int to "barely" add +1 or +2 damage to Summoner DoT ticks. Facepalm.