I think I know where the panel switching idea comes from.
If it was me, I would get get a bigger grid-tied inverter, one array, and let the geyser warm up during an optimal solar schedule.
Inverter + panels + GW timer or some such.
vs
GW Dual + inverter + panels
Bet the GW dual will cost a lot more than the bare-bones timer, even with the Tuya interface.
And I bet the bigger array, would safe you more in the long run on the house overall.
KIS
I have 2x 150L geysers in opposite ends of the house. When I got solar, I had the elements in both replaced with 2kW elements.
Currently what work for me is, 1st geyser turn on 10:35am. As soon as the power usage on it drops for 60 seconds, the 2nd one turns on. That is then for the day. The geyser where we take morning showers, will turn on for 1 hour in the morning for a boost on days where the average temperature outside for the previous 24 hours is below 17 degrees. This is enough heat for 2 people to shower in the morning.
I replaced the isolator at each geyser with the CBI Astute Isolator. This runs Tuya, but since I hate tuya, I opened it up and replaced the controller chip with an ESP12S and programmed it with Tasmota so that it runs MQTT to home assistant which now controls the geysers.
Some days they both finished at 1pm, other days they have to run through to 4pm if loadshedding happened at 10-12 or 12-2 and other crazy conditions. All of this is managed a ton of automations and conditions in home assistant. The morning boost, the inverter DoD will be switched to force it from grid instead of battery depending on conditions as how much sun is forecasted for the day or if battery is just too plain low etc. Took a helluva lot of time to tweak to work perfect, and there may be 1 tiny issue left, but its all working mostly now, around loadshedding when the geysers which are on non-essential cant power on.
This power source selector idea is on my todo list for days I know it could potentially run from sun, such as cape town summer fully sunny day and there is still loadshedding.