I don't play MM, so I can't put together an opener for you, but I can help with some guidelines...
1. Try and maximize casts of non-instant(Aimed/Steady) under Rapid Fire rather than instants. This does not mean to ignore instants(as Chimaera shot is higher priority than aimed even in Careful Aim range), but make sure your instants are on cooldown, and don't spend time under Rapid Fire using other cooldowns.
2. Do not activate Rapid Fire at high focus, as you'll cap out quickly.
3. Maximize Chimaera Shot/Glaive Toss with Readiness(this is not really MM-specific, but many people forget that we can fire signatures twice in a row)
I have my own opener, no idea if it's the most efficient, but it works: SrS - CS - GT - RF - Stampede - (AMoC) - SS - SS, after that I cycle SS and AiS until RF is over (while still doing CS), then I wait for the next CS and Readiness, RF again and continue SS/AiS while keeping CS on cooldown and Steady Focus up, of course.
I usually ignore GT while opening, it's not perfect I guess, but otherwise get get button bloat in that phase of the game AMoC is situational, depending what talent I have specced for.
Just play it like survival but with the added steady shotting in pairs when the buff has 4 seconds left. I found that aimed shot really wasn't worth the bother hardcasting it's damage is too low and one tiny mistake like focus capping during careful aim will remove any dps gain from hardcasting it there and when it's done correctly it's only like 1-2k more burst, pointless.
I do prepot > chimera shot > serpent sting > crows and rapid fire > glaive toss > steady shot x2 for the buff > arcane shot until low > fervor > chimera shot > readiness > chimera shot > glaive toss > spam arcane shot > fervor when low > spam arcane shot > steady shot for more focus and only do it 2x when theres around 4-5 seconds left of the buff etc.
It feels very much like the other hunter specs when played like this and you just use aimed shot when it procs. The dps difference from hardcasting aimed was nothing.