Hello!
I’ve done a very similar thing to my apartment, so can share some experience.
- Is Cat6 to switches, 240VAC to lights, relays a recommendable approach?
I would say “yes”. Yes, you can’t revert to oldstyle plain electric installation any more, but… Who cares about the past; it’s 21st century! I did so.
- A lot of people seem to be concerned that they can’t switch lights on if WIFI breaks down … This really mostly happens when power out … then the lights don’t work anyway … thoughts?
Well, in my apt i even reserved some wiring for a UPS, which could power the automation and maybe some lights i would consider emergency. Not using it still; power outages are extremely rare in my location. You may have a different situation though. In this case your solution, i suppose, would depend on how bad the reality is plus how deep your pocket is. Anything from a medium sized UPS up to a full-blown emergency gasoline/diesel generator.
Just try to avoid dependency on any clouds, a good system (IMHO) is a self-sufficient one. Yes, unfortunately such solutions may not always be available.
One more good bit of redundancy is to have some basic local controls. My modbus relays have external inputs, to which pushbuttons can be hooked; so even if the whole automation system is down, i can switch on and off my lights, and control all the equipment in a traditional way. You can try to find something like that.
- Any guides on sizing the switching cabinet? At this stage I would cater for relays, safety switches / circuit breakers, 24VDC PSU.
The bigger the better. 
My apartment (just two rooms + kitchen, 52 square meters total) fills up a single ABB UK540 box (56 modules) only with AC breakers, RCDs and consumption monitor. The whole install, with all the low voltage stuff, takes three of these boxes, mounted on top of one another. One more box with all the breakouts (TV, doorphone, Ethernet) and smart home HW and one more box hosting 12V power supplies for the whole system. These are the largest consumer-grade boxes i could find. Another advantage is that all the DIN mounts and faceplates can be removed, so you can leave a bare empty box, this is preferred for low voltage because the majority of the HW isn’t DIN-mountable.
Here is description of these boxes, for your reference: https://library.e.abb.com/public/ed9b8e347c054a53c1256afc0057c440/Serie%20UK%20500.pdf . I don’t know if they are available in Australia, OTOH; you may use American standards which i, frankly speaking, don’t know; i only know they are very different from european ones; and somebody might even think they are too restrictive and outdated.
If i was building the whole house, i’d go with a dedicated server room with a proper 19" rack, frankly speaking. This would give me a to-o-o-o-n of freedom.
- If you centralise lighting to switch board how do you handle dimmers? Any good suggestions?
First of all, don’t centralize. It will be a nightmare to install and cabling would cost a fortune. My setup is distributed; it uses one modbus relay (3-4 channels) per room. If you think well, there are plenty of places to hide the hardware. Think furniture, wardrobes, kitchen closets, etc. These modules sit on a single modbus line; it’s just one cable. Should you choose another technology (e. g. CAN, KNX, whatever), the idea stays the same.
As to dimmers… Frankly speaking, oldstyle dimmers, which plug in instead of a conventional switch in a series with the bulb, completely suck (IMHO) with modern lighting. If you want a proof, go to a local IKEA and play with the demo stand. You can’t dim down below 50% (and 50% is still pretty bright), so that’s useless. I’d say, if you want good dimming, your light sources have to be made with the proper support. The modern LED light source has a built-in power supply; and a good dimming effect can only be acheved if the power supply has such a function. I. e. you don’t try to cut the power to it; but it has some kind of control input; and adjusts LEDs accordingly.
One example would be Hue bulbs. Many modern LED chandeliers are sold with RF 430 mhz remote controls; they can dim and change color temperature; you could use an RF transmitter to control them from the openhab. But there’s a drawback: if someone changes the state with an original remote control, your smart home system won’t notice it because the RF channel is unidirectional. A good solution would be to replace the built-in PSU of such a chandelier with some other model, which supports e. g. 0-10V interface, but unfortunately i don’t have a success story here. This market looks like has just started developing.
- I plan to use Synology NAS as OpenHAB platform. Better to use Rasberry PI?
I don’t know Synology’s capabilities, but Java is a pretty hungry thing. Initially i tried running on 2GB Orange PI, that was slow and problematic; eventually i upgraded to rasbperry pi IV 4GB RAM.
Also some time ago i had some arguments here; and basically i was told that an RPi is a toy; and a “real server” would be far better. You know, if i had my personal family house instead of a small old apartment; i’d do just that. You can easily afford a small room with a huge industrial grade switch box on the wall and a 19" rack; UPSes and other stuff.
If you’re interested, i can share some photos of the system and good solutions i found.
Forgive me if not.
