I am painfully aware of the sun … it is my nemesis and greatest joy.
But holy smokes (pun intended) @Phil.g00, THAT CBI makes a damn lot of sense!!!
Now I’m having to rethink it all … just as I thought “I’ve got it!” 
Let me see if I can find out “why not” … I always start there, more difficult than to justify why.
The geyserwises are an expensive addition to what otherwise could be achieved by a cheaper single timer with battery backup.
Those CBI’s are great and come in higher current rating.
They are conventionally used to not allow a geyser to run when the stove/ oven is being used.
Well thats the intended purpose of the 1 still fitted to my fathers DB.
Probably to ensure no high loads or overloading of the circuitry, specifically in older homes.
Its actually a nice to have on smaller inverter systems to keep load down.
2 x 150l geysers with Geyserwise, one Tuya controlled, one not.
Problem is that these two are on separate DB’s.
So there goes one idea.
One Tuya-controlled 150l geyser and one 50l geyser (no GW) are on the same DB.
That can work.
Just need to figure out how to control the GW geyser element, without km’s of wiring.
Another Idea - for the 2 GW geysers on 2 DB’s:
Shelly has a potential free 16a contact. Elements in the geysers are 2kw. Sorted.
Install a Shelly on each geyser between the GW controller and Geyserwise element live wire.
Then set GW to heat each geyser between 10am and 3pm.
150l GW geyser will switch off at GW temp setting, the 50l one heats until the thermostat stops it.
Using NodeRED on the Venus, switch the Shelly to close the circuit to the elements, based on parameters.
But, still, untidy.
I would not use that directly, but I’m somewhat averse to house fires. I would use that to switch a 30-40A contactor and run both the live and neutral of the geyser element through that.
Gonna plug the CBI Astute relay again: Samite & DIN rail compatible, 30A resistive switching, so perfect for geysers (and pool pumps – 10A inductive). It’s got power measurement too (so you can tell when the thermostat turns off to switch something else on), which you obviously don’t get with a contactor.
Only “downside” (to some) is that it’s Tuya.
The “smart isolator” is the same thing, but in a 50x100 isolator housing if you don’t want to mess around in the DB board.
Asked my favorite sparkie re. the 16a potential free contact. He said it is fine if the element is 2kw max.
50l geyser is 2kw. Victron display says so, checked switching the geyser on/off.
And I know the other geysers are 2kw, had their elements replaced to fit the solar system.
Titibit:
Sparkie is quite pedantic and keeps me in line when I have some hair-brained scheme. He refuses to budge on any regulations. And, he sees into the future.
Like me, naaa, no babies in this house ever, all our friends have big kids, don’t worry.
Him, nope, not doing that, not baby safe.
… there is a baby in the house now.
BUT, I do listen to all opinions/advice/experiences, and I do agree, more protections are always better.
BUT, I also look at risk mitigation vs increased risk due to more complexity/parts like when things get so intricate, that it becomes a risk if one is not around to explain.
And yes, there is the code for all installations, but dangit, we do complicate things quite fast with solar supply and whatnot.
Carefully I’m pondering on all the options, then to get it past the sparkie to install.
EDIT:
If it was not for the potential free contact, I would not have even considered it.
On top of that, @Gman has dunnit already, he is as pedantic.
EDIT 2:
And I have already had sparks on my desk, again, when, due to light, eyes not so sharp anymore, I missed a teeny strand of wire when I connected the Shelly to 220v AC … it still works … and I’m fine. 
Yes, this one (ECU) is the latest one.
But please explain how you’ll wire this up…
I find I can’t do without one.
How do you know what temperature the geyser is at?
I’m sure you can hack the control as well if one applies oneself…
And you can buy them second hand (Gumtree/Marketplace) for R500 ea.
In the same way, as in the old days, set your thermostat goldilocks style.
I’m considering that … rip out the GW’s, and go goldilocks style.
I’m sorry… but the 80s called and they want their black DB-board mount thingy back…
It really looks like something from three decades ago
So what is the modern equivalent?
It looks like I have stumped the team… 
Isn’t there a prize for this??
Just be aware that the Geyserwise expects the water temperature to rise by 4 degrees per hour if it has its output switched on, otherwise it throws an E4 error. You have to cycle its power to clear the error.
This means you can’t put a relay on its output side to override its behaviour, and if you put a relay before it then of course you can’t see the water temperature. It doesn’t sound like you plan to do either, but I thought I would mention it for the benefit of others reading this thread.
This error has bitten me: I thought I was being clever by putting a 1kW element in a 150 litre geyser. It takes about 40 minutes to raise the water temperature by 4 degrees, so if there’s a little bit of drawoff in the heating cycle I get greeted with an E4.
Anything less than 2kW, in my opinion, also starts to run into standing loss issues. Your geyser loses 2kWh of energy in a day to standing loss, and it takes the element two hours to put that back (before making any new hot water). That’s still okay, but if you go away for two weeks and turn it off, you need 8 hours to get the tank back up to temperature… which does not sit well with the wife if you return at 5PM and she wants to shower by 8…
Now THOSE are the teeny titbits that I patiently wait for on forums. They are few and far in between, sometimes one spends hours reading through testosterone-fueled replies, but o boy, when you hit these gems, they are priceless.
Thank you @PierreJ
Although the idea was to have the Geyserwise just sit there, not heating unless manually switched on, the idea being to bypass it completely and just switch the element on, I’m parking the 2 x 150l GW geysers for the moment.
I will experiment on the 50l stock standard one. What can possibly go wrong, I cannot let the smoke out of a geyser … can I? 
Already shorted 220v input on a Shelly, don’t ask, and it still works.
To get back to the topic (which I understand as intelligent load control) I came up with an idea I want to try:
I’ve had uphill from CoCT re installing a grid tie system (inverter not on the list etc.) so my plan now is to install a (compliant) baby system of 1.5kW with no storage. This should be enough to cover the power house uses on a sunny day minus the geyser.
The GW Dual will be paralleled on the same 4 x 350W panels and it will only be enabled when the inverter is idling. The switching will be done by a current switch on the inverter AC supply. This will switch the MPPT on/off (with it’s control signal input)
