Currently there are 2 types of storage water heater tank technology,

a. Enamel-coated technology - newer technology, better leak resistance, and cheaper. CENTON (example here - Shopee / Lazada) and ALPHA (example here - Shopee / Lazada) is using this technology.

b. Stainless-steel technology - old technology, more prone to leaking, and more expensive. JOVEN (example here - Shopee / Lazada) is using this technology.
I suggest u to go for CENTON. By far the best-selling and highest 5-star rated storage water heaters on Shopee, and cheapest amongst these 3 brands too.

A single 90L tank suffice even with guest/parents simultaneously using the 3 bathrooms. But since you rarely use all 3 bathrooms, can also consider fitting a standard tankless water heater to save energy.

For master / powder room / doggie shower room / jacuzzi, suggest u to go for 90L, since there's a jacuzzi involved.
Underutilized means on an daily basis, maybe at most u will only be using 50L of hot water, but u are boiling 90L of hot water. So u are not fully utilizing the remaining 40L of hot water.

Disadvantage is you are wasting electricity, but not significant if u are constantly using the hot water in the tank daily.
IMHO, no issue if u do not want to turn it off if u are using it on a daily basis. If the switch is kept turned on, the thermostat in the storage water heater turns on and off to keep the hot water in the tank (usually at bout 70 to 80°C, depending on brands), but temperature doesn't drop much due to the insulation around the tank to keep the heat in. If u turn off the switch, this thermostat turning on / off to keep the temperature constant won't happen, n u will save more energy at that point of time, but when u turn it on again on the same day, u have to wait for the water to boil again and require the close to the same amount of energy to boil it up. So the energy saving u'll get for turning off the switch is insignificant (again, if u use it on a daily basis), with the downside of having to wait for the water to boil up again.

But do turn it off if you won't be using it for a period of time, eg. the one in your guest room, or if u are away for a holiday.