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Please note, though, that this is not on general discussion of and advice on home automation - which is what you’re after.
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That being said, there’s already a number of threads on questions like this so please search the forum as a next step.
In general, centralizing cabling is a good idea if you can afford it (and you can since you’re building from scratch). Eventually you can make it not 1 place but maybe one per floor level. Having this in place will allow you for selecting and replacing your actuators / technology at any time, whatever that tech you select may be for now.
Thanks to openHAB you can select any number of technologies. In the long run you’ll end up with a combination of 2 or 3 technologies anyway, so why not start like that from the very beginning. Choose per-actuator whatever fits the use case best.
Tech to choose from I’d suggest would be KNX, ZWave and WiFi.
KNX is wired and good as a general-purpose system but pricey (and not sure about availability in S.Africa), but it isn’t good at lighting (dimmers or even colour)
WiFi is cheapest but comes at a price in terms of security, lack of standards and electrical quality, and the set of vendor/devices to choose from for lighting is very small, particularly when it comes to dimmers and color lighting.
ZWave is radio based. It’s also general-purpose, somewhat similar to but better than Insteon.
At least with KNX and ZWave your “still can switch if automation is down” will work.