When "that is" means "in other words," can it be used in colloquial speech? Or is it more suited to a formal register? Does the following sound natural in daily speech?
We, that is, my brother and I, are going there tomorrow.
To be Honest no it does not sound correct I believe you should have used the saying
***We, I should say, my brother and I, are ...
Academic style varies somewhat from journal to journal and from institute to institute. However, many large academic publishing houses (IEE, IOP, Elsevier) do not express preference on the usage or not of personal pronouns. Often research attempts to express objective truth, where personal pronouns would be misplaced. However it is unavoidably true that ...
Generally, in a formal paper the proper etiquette is to avoid first person words/point of view (I, me, we, my, our).
"This paper outlines..."
"This paper argues..."
"This paper demonstrates.."
The above are all valid for formal academic writing.