why would you not have a light toggle or all devices off toggle next to each group?
On the individual device tiles, if you tap the icon, it will toggle on/off, if you tap the tile, you get the controls.
on the top of the screen, in the new-ish row of device types, if you long press the lights group, the first option is "all lights off".
or if you click that, and go to the lights page, at the top if any lights are on you'll get a small tile with "<number> lights on" if you tap that, it will turn everything off. If you tap it again, it will restore the lights it just turned off (not sure how long after turning off it holds that though).
I believe you can do a bit more via their APIs but that doesn’t help if using the Home app.
Just because you're using the home app, doesn't mean you have to use only the home app. All of the alternatives just tie into the same back end, and things created in one, show up in the other.
Two popular options are eve (free) and controller for homekit (free, paid for some features).
Eve will allow you to add conditions to automations (you could probably do with shortcuts in the home app). Eve also lets you drag/drop or copy/paste settings from one light to another. Helpful when setting up scenes and you're trying to get a bunch of lights to match. You can also view by room or by type. So if you want to alter a scene, just open and go to the level page, and on one page, you have a fader for each bulb in that scene that you can adjust, instead of having to go through multiple tiles. Eve gives you some extra features when controlling eve devices, but you don't need any eve devices to download the app and use it.
Controller lets you copy an entire device to another. Last year I switched out some older Hue color bulbs, for the newer brighter version. I was able to clone the settings onto the new bulbs. so all of my scenes and automations just worked for the new bulbs without having to re-build them. (this is a paid feature)
I normally use the home app for day to day control (or Siri). Eve for creating/modifying scenes (plus it runs on Apple Silicon Macs). Controller mainly just that one time to clone those pictures,