Hi! I have heaps of experience working with Instagram, my portfolio includes but not limited to, Auto-Commenting on IG Posts, Finding and Removing Ghost Followers, Automating Posting of Photos and Stories for hundreds of account, etc. When we chat, I can show you a demo to show you that i speak what i do. As for your project, 1.5 Million messages to 1.5 Million Instagram users, it will not be a problem for me. I am sure, you don't want to get your main account flagged, moreover you understand this task can take ages with one account, therefore we need bot accounts to do the work for your main account.
First we would need to create bot accounts, for this we will need a proxy service.
Second we would need to run the bots to send messages, for this we will again need a proxy service. WE cannot send too many requests through same ip, else Instagram will block IP.
Third, we would need a place to run the End-Tool. I recommend AWS, we can use free-tier account.
The end-tool will send 1.5 Million Messages, without missing a single one.
For more discussion, lets chat.
Regards,
Hamza Zubair