What is the safest and most efficient way to move large sums of money between two European countries?

SEPA transfers work well. If you consider the amount significant, it would be a good idea to visit your bank branch and let them know of the transfer request. It would also be a good idea to read up on the answers of others here to be 100% sure you’re not doing the transfer because of some pig-butchering scam or something similar.

But to answer your question, a SEPA transfer is pretty secure.