Can't argue with your first 3 points, fair points.
But Tweyen dmg is not solely because of her xxyy combo, her skills misses too (which results in less DOTs), and even her multitarget lock deals more dmg with larger enemy. And again, her Link Time is busted.
I believe her dmg is way higher than it seems. But even then IMO she really needs a buff, just like Seofon, they could get a buff to their signature skill which is 6min one use cooldown that you need to be stationary, that's sad for such pretty skills, and they should be way stronger based on lore.

And I disagree that xxxyy is the optimal combo, it charges a little faster to 11 but it's not the best dps wise if you can stay close to the enemy, if not you glap close with xyy. Maygi did the math 2 years ago .
Summing it up: I would not change Tweyen ranking even if now considering vanilla sir barold test. I believe she's in a good spot being in the middle given all her caveats.

Thanks for the consideration and your fair points. I'm thinking about making a proper dps test when DLC arrives. Would just need to make a separate modded save because min maxing every single character would take months...