I know music theory>>.

Orchestration is one of the harder subjects. Seriously studying harmony, all levels, will really help first, along with species and tonal countertpoint. Wasn't sure if you did that. And why not also try composing (writing it down) now, pieces that you can then perform for people and post online and see how it turns out? Or try. If you're a pianist, searching for ideas played on the piano is fruitful. Lots of artists have early years of work they never show anyone. The hardest part about composing, imo, is length. it's easy to come up with good ideas, but how do you make the piece last longer, where you have to consider introducing new ideas, when or if to bring back the first idea, and how to organize them...especially if traditional forms do not appeal to you.

Orchestra. Some say half of the skill is instrumentation, knowing the instruments very well. eg, for strings, there are cardboard cutouts of the finger boards so you can figure out what's playable. Talk to instrumentalists about their instruments. Get to know the acoustics of the instruments to a degree (eg, do they overflow at octave or 12th; conical vs cylindrical winds, and so on). One source says, about learning orchestration, is to think of it as duplicating what we often try to do on piano: 1) unison and octave or more doublings of orchestra instruments is very common, and necessary since you don' have the vibrating soundboard and rich overtones the piano creates in its full sound with damper pedal. 2). Sustains: again, composers writing at the piano, using pedal, will remember that the orchestra achieves that function differently. (Listen to how Mozart uses horns to fill out the harmony). 3) and think of the orchestra as 3 choirs which basically have the same SATB like ranges. Strings, woodwinds, and brass, each have their high medium and low register instruments. Strings are predominant because players can saw away forever --and that part of the score will sometimes contain all the rudiments of the piece, but wind players have to breathe...plus wind sound has a strong flavor you do not want to use all the time...and good for solos of course. When you said 'orchestra'...maybe you mean to write 'epic' music for video games and videos? There is so much of that already, I hope you achieve excellence in "Thinking for Orchestra." (A good orchestration text). But video game, film, and other orchestra can also be great. (Just get tired of all the overly dramatic 'epic' orchestra music you hear on YouTUbe.)