If you can let STM do most of the work, see if that works for you! The AutoWrap feature sounds like what you're after, with VerticalLimit being its vertical counterpart. (On UI text, these values are controlled by the size of the recttransform, by the way!!! Unless"ui wrap" and "ui limit" are disabled.)
The STMMatchRect code in my sample scenes really sounds like what you're looking for - it just takes an existing rectTransform which you can change the appearance of, and resizes it based on text size. (With options for which bounds it's copying.) So please check out the sample scene for that.
What is your monitor's refresh rate? If it's higher than 60, that means the issue shouldn't be something delta-related, so that helps narrow it down at least. (Thinking it's happening on a per-frame basis, rather than a per-time basis)
Got your zip, will check it out shortly!