How can I start bitcoind without blockchain synchronization? Suppose I make a locked wallet with 50000 pre-generated addresses.

As long as I researched, only way I can collect those 50000 addresses using RPC and "getnewaddress". Feel free to correct me.

I understand this will took several hours or days, so I can set this up on some VPS and let it run.

For this I do not need blockchain to be synchronized.

How can I start bitcoind without blockchain synchronization?

I am thinking about "connect" option or testnet, but both looks like a hack.
The recommended way is to use the -connect option. You can do -connect=0 or -noconnect (they mean the same thing) and Bitcoin Core will not try to automatically connect to any other node.