App submission stuck in "pending" for ~3 weeks

Unfortunately, not much will happen. I had the same problem with an app and after some searching, I found the cause. You have a manifest where you specify which watches are compatible with your app, and that's where the problem lies. Their automation probably indicates that there are issues with at least one of the watches you've listed, but that doesn't mean you'll be notified. Here's how I solved it: I own several Garmin watches, and as a first step, I physically tested the app on these watches (Fenix ​​8, Forerunner 255m). I learned that the emulator ran smoothly, but there were problems on the watches themselves. I fixed that, added only those two watches to the manifest, and submitted it to the store: voila – it went through. After that, I categorized the manifest step by step into product families and discovered that, in my case, the older Instinct models were causing the problems. I hope this helps you.

that sounds strange. Are you sure you added back all other devices except the old instincts? 

In my special case there were not only the old instinct watches but all watches leaking enough video memory.  It really depends on what you are building.