[nikitoci]

You are not limited by 6-digit passcodes only, you can also

“…Or tap Passcode Options to switch to a four-digit numeric code, a custom numeric code or a custom alphanumeric code.” which is on their support web site[1]

[1]: https://support.apple.com/en-gb/HT204060

[lxgr]

Yes, but then I need to enter a custom alphanumeric password every time I unlock my phone or tablet.

I want to be asked for it if and only if I grant a new device access to my end-to-end encrypted iCloud data.

I don't think this is an absurd demand. WhatsApp supports this security model, for example. Evem Apple used to, before they forced every iCloud keychain user to switch to their HSM-based model!

[SllX]

Do you not use FaceID or TouchID or unlock with the Watch?

I switched my pin to alphanumeric because I’m not putting it in every time I pickup my phone. I can live with the inconvenience of putting the passcode in every couple of days or so.

[brookst]

I see what you're asking for, but I don't think Apple would ever do it. A passphrase that is only used once every few years is a recipe for endless support calls.

[ace2358]

I’m aussiming you don’t use Touch ID or Face ID?

I’ve been using an alphanumeric passcode for about 7 years now. I’ve gotten used to it. It’s not too long to be annoying but better than a numerical pin.

Even if you used 4 numbers for an alphanumeric password, it’s still much more secure than a 6 digit pin.

[brandonbloom]

It's not exactly what you want, but one mitigating factor is if you're using FaceID, TouchID, or Apple Watch -- Those things will dramatically reduce the frequency that you're prompted for your password.