millions of Bitcoin addresses worked for personal use for a project without mentioning what it is and linking addresses to a single wallet
The only reason I can think someone would need millions of addresses is spam/scam/extortion email, where you'd like to keep track of who paid you.
So I'm curious what your "personal project" involves.
That being said: have you thought about how you're going to keep track of which address got funded? If you import a few million addresses into a wallet, I expect it to grind to a halt. You may like
this solution.