Yes, I think KNX should handle as much workload as possible, so your sensor devices should send notifications on events such as button press and actuators should react to these button presses accordingly in KNX without need to use HA. KNX devices are expensive and they should work for what we pay for them.
You also can set currently unused KNX sensor buttons to send telegrams to KNX bus to have an ability to implement custom functions in HA later.
Regarding your question about activating light by movement and by switch:
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Activation of light by movement in KNX is usually called “Staircase function”. And that is a feature that a particular KNX dimming or switching actuator model might have or might have not. Usually dimming and switching actuators have this function.
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Whether it is possible or not to combine both “Staircase function” and manual switching using wall switch/button (you call it master switch) using the same actuator output - I am not sure, you should consult your KNX system installer that. But in general, you can always use two separate actuator outputs to power the same light.
Regarding your question about irrigation idle on rainy day:
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With KNX I do not think that is possible to get weather forecasts at all. The only thing I think you can get from KNX weather station (that has rain sensor) is whether it is currently raining or not. But no forecast.
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On the other hand in HA, with at least some weather providers it should be possible to know forecast and use it for automations. I didn’t do it myself yet but in this forum you can find how people are pulling out forecast attributes and use them for automations, e.g. Rain forecast automation - #11 by Amaia_Espejo. And as author of that thread put - that might require some “witchcraft”
