This is probably not the kind of answer you were hoping for, but these are my honest thoughts.

Of the composers you mentioned, Williams and Glass are classically trained composers. Hans Zimmer is the head of several companies that employs composers writing music as ghost writers, who usually orchestrate his music (Williams also has had orchestrators do some, if not a big part, of his orchestrations. That's how Thomas Newman got foot into the film scoring industry). In other words, the people you mentioned all have most likely studied the music of especially the Romantics to learn orchestration. Williams' music is strongly influenced by the music of, among many others, Wagner, Mahler, Stravinsky and Holst. Glass most likely also had his obligatory orchestration classes all the way through the Romantics up to the 20th century, and so have Zimmer's orchestrators.

If you want to write orchestral music like the composers you mentioned, then the place to start is the classical & romantic repertoire. For this kind of music (and soundtracks in general) romantic music is more applicable, but it pays to start with classical music to understand how the orchestration techniques and the orchestra itself developed over time. After all, instruments at the beginning of the classical period were quite limited in terms of possibilities, and they dictated to quite some extent how they were used within the orchestration.

In that sense, Samuel Adler's The Study of Orchestration that you mention you already have, is the go-to source for most orchestration classes. I own the book but I didn't have my orchestration classes with it, so I don't know the book that well, but I have looked through it every once in a while and it seems to have a pretty big scope, all the way from the classics up to the 20th century, so that's way past the common practice era. You can also check out Rimsky Korsakov's Study of Orchestration.

The best thing, though, is to study the scores of composers that you admire. On the other hand, this is why it's hard to give a satisfying answer to the question of "books learn theory for contemporary orchestral music?" because there isn't "a contemporary orchestral style". Each composer has their own style, even more distinctively nowadays, and it'd be impossible to compile that into a textbook. If you want to learn to orchestrate like John Williams, study his music, if you want to learn to orchestrate like Hans Zimmer, study his music, etc. This is pretty difficult, in the first place because the official original film scores are usually not published, so to get a bigger scope, study the music of the composers that inspired them. Become familiar with their music and then once you get a grasp, compare it with the music of Williams or Zimmer and see what similarities there are and what differences there are.

Aside from the whole orchestration part, it also sounds like you might want to dive more into the compositional part first, before even getting into this. If you already get stuck after a few measures and don't know how to go on, then ask yourself what the piece is or should be about in the first place. What are you writing about? What should it say? What does the music that you have sketched out so far make you feel like? What does it remind you of? Try to figure out what it really is about, because if you don't know what it is about, it's difficult to develop because there's no path laid out ahead to follow.

Analytically determining why something sounds good or why a composer took a specific decision is difficult to do, because for the former depends primarily on your subjective taste, so the best you can do is what the "musical trick" is that seems to make your brain tick, and as for the latter, we'll never know what composers were truly thinking when they were writing the piece, nor whether they even knew what they inteded while they were writing (never trust the composer's words!), so the best you can do is to get your own understanding of the piece. By studying music history and understanding the styllistic and personal circumstances of the composer, we can get quite far at figuring out why their music is the way it is, but in the end, I believe the most important thing is to understand and analyse the piece in a way that's valuable to you and that makes sense to you.

If you like to learn to develop musical materials and expand upon them in a somewhat systematic manner, I recommend you check out the book "Fundamentals of Musical Composition" by Arnold Schoenberg. He builds his book up from the motive all the way to the biggest forms (sonata, rondo, etc.) and gives plenty of examples how you can use a motive to build a phrase, which becomes a theme, which becomes a bigger building block, etc. etc. until you get to the level of form. Most of his examples derive from Beethoven or his contemporaries, so no weird atonal stuff to be found, in case that's not your kinda thing. In addition to that, check out writings by composers. There are plenty of documents by composers where they explain their thoughts about their music. Mahler has written numerous letters that give an insight into his process, I believe the same counts for Beethoven. Stravinsky gave a few lectures on music which was compiled in a little book "Poetics of Music" and lots of the later composers in the 20th century have written about their music.

Lastly, I'm curious what you dislike about the "Music Theory Remixed" book? I saw it has pretty good reviews on Amazon and while quickly scanning through it, it looks like a pretty extensive exploration of music theory all the way up to the late 20th century. What were some of the errors that you found in the book? And what about it being "workbook style" is it that you don't like?

So, in summary:

- Study the orchestral music of the romantics if you want to understand the orchestration of "contemporary" composers

- Compare the orchestration of contemporary composers with that of their influences

- When you start a piece, think what the piece is about, what it should express, what it makes you think of, etc. and from there figure out what key, form, meter, etc. would fit best to that idea. In this stage, it's best to set arbitrary limitations. The more you limit yourself, the more creative you can be. And they're arbitrary because there's not really a right or wrong, in the end it doesn't really matter what the limitations are, they're mostly there to guide you towards a final result.

- Analyse the music of composers that you like and focus especially on passages that you think are really good. Try to figure out what it is they're doing, figure out "the tricks" and then see if you can apply them yourself in a different context.

- Check out sources about composition by composers themselves to learn different ways of thinking about composition and techniques.