Am I right to think that if I buy this TV, I won't be able to do the same without an external ATV this time being a Apple TV 4K.
No reason why your old HD Apple TV shouldn't work with a 4k TV - you just won't get 4k or HDR. Instead you'll get HD upsampled to 4k which is usually pretty good - and, of course, that's all you'll get
anyway if the content you're watching isn't 4k or HDR to start with. Remember that with some services (e.g. Netflix) you have to pay extra for 4k.
...and whatever TV you're looking at, you'll need to confirm whether the built in Apps support 4k, anyway. On the flipside, even if the TV doesn't have AirPlay, it will probably have apps for all the common services, which ought to be more efficient than going via your iPad & AirPlay (meaning 2 trips over your WiFi network).
I'd pick your TV based on picture quality, price, size and availability & just make sure it has HDMI inputs and eARC support so that you can supplement it with current and future streaming devices. If the TVs own apps turn out to get the job done then fine - but you're quite likely to end up wanting an ATV 4k or other streaming box
anyway sooner or later, so I wouldn't pick a TV based on its Apps.
I've recently got a LG "WebOS" TV and the built-in apps, including AirPlay, NetFlix and Prime, seem perfectly good - but I'd already got an ATV 4k to run things like MrMC (the App Store friendly fork of Kodi) and do iTunes 'home sharing' with my yet-to-be-BigSurified Mac. The UI on the AppleTV runs more smoothly than the TV Apps and maybe (
maybe) the AppleTV gives a marginally better picture (not sure) but I probably wouldn't bother with the AppleTV if it wasn't for the other things... except one dealbreaker: the LG doesn't support DTS sound - only Dolby - and the LG media player won't play the sound on some files stored on my local sever. The ATV will transcode these to Dolby and they play fine in MrMC etc.
...flipside, the LG Apps include a perfectly usable and armchar-compatible web browser - Apple seems to think would bring about the end times if they included Safari on the Apple TV.
I do find all of this redundancy annoying (...do I stream from my iPad, use the TV Apps, use the Apple TV, when they all produce a perfectly cromulent result) and it's frustrating that I can't just buy a dumb display that doesn't want to talk to the Internet, with a good picture and a bunch of HDMI inputs for the actual content sources....