It was easy to index a million URLs in the past, it is going to be harder.
You should have dripped them. You should have drip-fed them.
Now that it is all there, here is something which may have hypothetically (

)worked for me in the not-recent (!) past:
Create your sitemap with a few thousand URLs. Create a rolling sitemap daily, and submit it with a few new URLs at the top. Make sure they keep getting indexed. After every two weeks or so, gradually increase the number of new URLs. Remove the old ones which are already indexed from the sitemap.
Keep doing this.
Most important tip:
Adjust based on what you observe. Do not do things without thinking. Read between my lines. Assess your situation.