1) Every service most likely has many, many different servers providing it. These may have different hostnames. Particularly consider that Apple uses a lot of cloud services.

2) All the services you list in fact consist of many smaller services. iTunes alone is a storefront, a CDN, a payment processing service, a DRM system, a syndication service, an account management service, a media library synchronisation service, a streaming media service, and so on. iCloud is a blanket name for a large collection of big services which themselves may consist of many smaller services.

3) That isn’t even the full set of services.

In light of these considerations, 1000 hostnames should not be unexpected. That might even be a surprisingly small figure.

Why is it suspicious? I don't see your reasoning here. Having a diverse range of DNS names requested doesn't seem to me to be an indicator of suspiciousness.

On earlier releases of iOS that had a public jailbreak released, I spent quite some time using HttPeek (https://github.com/Yonsm/HttPeek) to examine what various OS processes were sending, and found nothing untoward.

I'd be surprised if this had changed for the worse in more recent iOS releases.

One area where Apple uses a large amount of domain names on purpose is for their captive portal detection. Supposedly they do this on purpose so that captive portals can't try to hard-code a list of domains in order to white-list/fool it.

Why not?

I've developed some very large globe-spanning systems that are probably a single-digit % complexity as the Apple ecosystem and we touch hundreds of endpoints. Doesn't seem suspicious to me at all.