TL;DR While your private photos may be safe, WhatsApp and, by extension, all Meta services know you sent something, when you sent it, and who you sent it to.
"However, when it comes to sending private photos, there’s an even better way than messaging apps...#ProtonDrive."
Please explain, why ProtonDrive is more secure sharing photos than #Threema Libre. (Apart from the expiration date.
A password only helps, if the end-users' smartphone enables it. Once downloaded to the phone, it's as secure or less (you will always download a pic from a drive, whereas you don't need to in the #Threema timeline.)