You need an AppleTV or HomePod to act as the hub. Without that, you get no automation nor ability to control devices from outside your home. An AppleTV is also needed for Thread support. Using an iPad for a hub is deprecated and not supported in the new architecture.
You can of course use HomeKit devices with Home Assistant in both directions. HA can obviously serve as the automation hub for HK devices ("accessories" in HK parlance) and HA can expose itself to HK so you can control HA devices through an iPhone, iPad or Mac.
Ah, thanks for the correction on that. Maybe plenty of folks are happy with HomePods for their hub but all the complaints I read are also folks using HomePods. It's hard to know whether they have spotty WiFi or if HomePods are just inherently less reliable hubs.
I will say that having my wired AppleTV act as a hub, I've had no trouble at all. I have maybe 50 accessories and a bunch of automations, some fairly complex. e.g. my garage door automatically opens for me as I near my home as long as my iPhone is in CarPlay mode.
You can of course use HomeKit devices with Home Assistant in both directions. HA can obviously serve as the automation hub for HK devices ("accessories" in HK parlance) and HA can expose itself to HK so you can control HA devices through an iPhone, iPad or Mac.