Help this average/bad demo lock

So long story short, I've been maining a hunter for years, but this season I sacrificed myself for the team and I'm rolling with a lock as my main for raiding with the guild since we had no lock and plenty of hunters. I used to main a lock back in vanilla/BC/etc, but the class changed so much you might as well call it a new class.

So I've been doing keys and raid with the lock, I try and fill my vault both with heroic raid and M+ stuff, currently at ~2130 io, 405ish gear level. Nothing too fancy, I know, my hunter is actually significantly higher, but that's besides the point.

My problem is while I can do keys (doing 15-16s fairly well) and manage to down bosses in heroic VOTI (we're 7/8H right now starting work on Raz), my numbers are fairly shit. I do really well on big AoE packs but my single target performance or performance when there's just 2-3 enemies is just... bleh.

Here are the logs I have, if it's any help: https://www.warcraftlogs.com/character/us/kargath/felsworn

On mythics I typically run a cleave build (tried single and getting similar overall results) and I'm hitting like 40k average on most, which is usually bottom of the meter with comparable ilvl players. My hunter is slightly better geared and doing 55k average usually and while it's good, I've seen other classes blow my hunter out of water, let alone my lock.

Every now and then I'll run a mythic with a demo lock and they'll have absolute bunker numbers and I legit don't even see how it's possible...

Any help would be appreciated. Constructive criticism please, I know I'm pretty shit I don't need to be told :P