WhatsApp, Signal, & some others use the same open standardization end-to-end encryption. It’s the bare minimum bar for acceptable (but most of these apps require a primary Android/iOS device to hold the key which plays right into that duopoly as well as making smart phones a requirement rather than optional).
Discord has no e2ee, many rooms require phone numbers, the service is proprietary, they have trackers, they send cease & desists to projects wanting to be alternate clients. These are not the hallmarks for privacy or security.
Conversations is paid and has like 100k downloads, and it looks like it's from Android kitkat. The other two don't even exist on the app store. Do you consider these to be popular? I'm looking for actual popular apps, just like I can say Element for Matrix.