Sync first, unplug your internet, open your wallet, wait a bit. Keep it offline
ah ok, so whatever happens after node is synchronized and upon loading a wallet, does not require pulling new data from the Internet. Good. I will need to go online later on anyway to actually broadcast the transaction but that's ok - I was worried about keeping a node with loaded wallet exposed to the world for many hours or even days during synchronization or reindexing (or whatever is happening there).