scarlet spade
I'm clearly doing some pretty terrible things, going by my ilvl parse. Here's the link to heroic Vorasius: https://www.warcraftlogs.com/reports/Kj3nMft2CFzBmrk9?fight=31&type=damage-done&source=8
I'm parsing equally poorly on other fights, but I figured a nearly-pure single target fight would be easiest to diagnose. Any help is appreciated!
trim umbra
Don't put too much weight into ilvl parse, for what it's worth. Not all ilvl is the same (big sword gives more power than big wrist), so it's a very shaky metric that I wouldn't care about.
I'll dig in here and see what I can find to help you out 
So the first thing i'd mention is Talents. It looks like had a bit of a Slayer AoE build going on, since you took both Broad Strokes and Powerful Momentum. Less than 15% of top logs take those talents, so i wouldn't recommend it. Luckily, neither are needed on this fight because the adds die so fast, so you can just rely on your baseline Sweeping Strikes and it should give you more than enough AoE for padding, while letting you move those talents elsewhere to give you more boss damage. For example, both can go into Bloodborne instead.
Sudden Death is huge for Slayer. Every time you use it, it lowers the cooldown of your Bladestorm by 5sec, and does a bunch of other cool stuff for you. Looking over your Sudden Death buff, It looks like there was a few times throughout the fight where 12-20 seconds were passing inbetween Execute casts when you had Sudden Death available.
For context, Slayer's rotation is basically Major Cooldowns > Mortal Strike > Sudden Death, so outside of your Bladestorm channeling it's basically MS > Sudden Death, meaning it should be consumed within a few GCDs (not 10+).
When the boss hits 35%, you want to change your rotation into the "Execute rotation". During this phase, you only want to cast Mortal Strike if you've cast 2x Executes first. This stacks a buff on you called Executioner's Precision, which massively increases the damage of Mortal Strike, but caps at 2 stacks.
Looking over your Heroic Vorasius kill, you're often sending Mortal Strike with only 1 stack, as seen here:
Other than that it's all small things, like making sure you never have downtime (follow the ABCs or "Always Be Casting"), and making sure that Colossus Smash is always cast before Bladestorm
I'd start there, and see how much that helps, then give it another go 🎉
you got this 
scarlet spade
yooo this was a lot of effort, really appreciate all this! what a legend
trim umbra
Of course! It's what im here for. Let me know if you have any other arms questions, we're here to help 🎉
scarlet spade
Circling back on this - did normal March tonight and these tips made a big difference. Still more to work on (esp cooldown timings on some fights), but it's feeling great! Thank you again Hrothmar ❤️ https://www.warcraftlogs.com/reports/wbPRjvGx8nDWMNXd?fight=15&type=damage-done