This drives me insane. It's 2024, no iPad support is inexcusable. I'm an iOS developer, I've been doing it since the SDK was released to the public, it's not hard to support both interfaces especially if it was an iPhone app first, and I say this as someone who has to do some crazy complex interfaces at work and often needs to make a separate UI just for the phone.
No native Apple Watch app either, so we have to rely on 3rd party apps which aren't great. For an app that's used as much as this one, it's honestly shocking there is no native iPad and Watch app.
You have no complaint. If WhatsApp is important to you, get a new phone. If not, then you have no problem either. The iPhone 6 series was released over 10 years ago. Nothing is permanent, not even you. Namaste