How can I diagnose a fuel system issue without taking the whole system apart?

I have a 2017 WRX with 12k miles on it. It's still pretty new. Recently, when I turn the car on with a cold start, the RPM fluctuates heavily from 2k to 300 then back to 2k, then to 1k etc. It's a very noticeable sound as well since I have an exhaust. This doesn't happen all the time but it happens enough for it to be a concern. When it does happen, there will be almost no torque or acceleration until about 3k rpm when a VERY noticeable kick of power hits and the car accelerates properly. When I turn it on from a warm start, it never has this problem. Only from cold starts will it do this. I plugged my AP in to monitor my AF corrections and what not and the numbers seemed normal from the AP. To be fair, this was when it was a warm start so it wasn't tripping out like it did from a cold start. I haven't gotten a chance to look at the AF corrections from the bad cold start yet but I already know it's a fuel problem. I don't know if it's a clogged line, bad throttle body, busted injector etc. All I know is the engine is having a hard time getting fuel into it sometimes. I know it's not a spark plug issue because there's no engine knock. Plus, it starts up every time so that kind of eliminates a spark plug issue to begin with. I want to figure out how to diagnose which part is acting up without having to take apart the car and inspect each part individually. Any idea how I can effectively do this? Or do I have to bite the bullet and take it to a garage/take it apart myself and see what's up?

My warranty is voided since I have an intake and a tune on it so I don't think taking it to Subaru is going to do me much good. I plan on getting a catless DP soon and want to fix this issue before I do anything else to it just so nothing goes wrong during the tuning process.