There are a few standard sources for learning about linear algebraic groups, such as Humphreys' book "Linear Algebraic Groups" and Borel's book by the same title. Both are not written in the language of group schemes. Humphreys does everything over an algebraically closed field, whereas Borel already introduces fields of definition and studies rationality properties through the text. This makes Borel's book more comprehensive but unfortunately the older language of algebraic geometry and in particular $k$-structures is a bit disconcerting especially if one is used to and prefers the language of schemes and functor of points.

Hence this reasoning led me to choose Humphreys as a source to learn this material, who only works over an algebraically closed field, which has the advantage that the language difference isn't really an issue.

I'd read through parts of Borel on things that I need, but now I feel like I'd like to have a deeper understanding about rationality, $\overline{k}/k$ forms of groups over some base field, etc. without having to go through Borel since the language is awkward from my point of view. So, I'm looking for references:

An ideal reference would cover the following material, via the language of schemes and group schemes: everything that Borel (or Springer, in his book) does on rationality and fields of definition, and perhaps some more recent results as well, seasoned with plenty of Galois cohomology. Some elegant papers using the group scheme language could also be a substitute.

In other words:

Are there any good references on the basic rationality properties (such as the existence of maximal tori and Borel subgroups defined over the base field) for someone who as already read the basic material covered in a book like Humphreys, that uses the language of schemes?

I don't have too high hopes for a book, but perhaps there is some survey paper I've missed?

Note that Question 17662 is different in that I'm not looking for a book that covers all of the classification of reductive groups; for this, Humphrey's book is fine together with Conrad's fine notes.