My sitemap indexing is stuck in Google Search Console!

Okay, I am seriously pulling my hair out here. I just launched my new SaaS a couple of weeks ago, and I'm facing an absolutely infuriating problem with sitemap indexing in Google Search Console. My pages are just not getting indexed, and it's killing me because I need this content live and discoverable ASAP!

My site is built with Next.js, hosted on Vercel (with some assets on AWS S3), and has about 500 dynamic product pages. I generated a clean XML sitemap (https://myapp.com/sitemap.xml) and submitted it to GSC days ago, then weeks. The problem is, either the sitemap itself shows errors, or it just sits there with "Couldn't fetch" or "Sitemap could not be read" even though it loads perfectly in a browser. And even when it occasionally shows "Success" for a brief moment, the number of indexed pages remains stubbornly at zero or a tiny fraction of what's in the sitemap. It's like Googlebot is completely ignoring my sitemap, or there's some deeper issue affecting my crawl budget.

I've gone through every troubleshooting step I can think of:

• Validated the sitemap XML structure multiple times using various online validators – it's perfectly valid.
• Checked my robots.txt (https://myapp.com/robots.txt) to ensure the sitemap is explicitly listed and nothing critical is blocked. It's clean.
• Ensured all URLs listed in the sitemap return a 200 OK status code. I've manually checked a good sample.
• Resubmitted the sitemap in Google Search Console multiple times, hoping to kickstart it. No luck.
• Checked server access logs for Googlebot activity (limited success/no clear issues, seems to visit robots.txt but not consistently the sitemap itself).
• Verified canonical tags are correctly implemented on all pages, pointing to the self-referencing URL.

The most frustrating part is the contradictory feedback in GSC. Here’s what I often see:

Sitemap: https://myapp.com/sitemap.xml
Status: Couldn't fetch
Last read: N/A
Type: Sitemap
Errors:
  - General HTTP error: 404 (Not Found) - (Even though it loads fine in browser and returns 200!)
  - Sitemap could not be read

How can it be a 404 when I can open it in any browser instantly? This is driving me absolutely insane. Is there some obscure Vercel setting, a DNS configuration, or some advanced GSC trick I'm missing? Could it be a subtle crawl budget issue I'm overlooking for a new site? I'm completely stuck and need some fresh eyes on this. Any unconventional tips or debugging steps would be a lifesaver.

Help a brother out please...