Not worth, can't find spare part, buy new one cheaper.

water flow weak  most probably pipes are block partially due to old pipes / house  , to confirm need to check if the  water input to WH  is strong
or the WH itself  . the  mesh filter at the input control  may be clog of sand particles . need cleaning.

if  your water input is strong flow  then  get the dh-3m  , non pump.

pana WH do have 15mA elcb built in .
so a RCCB specially for WH  is optional up to you /budget.

ceo684 Is this the way to find certified electrician to check and install apartment DB and wiring?

https://ecos.st.gov.my/ms/web/guest/senarai...aftar-elektrik-

Last week i call to those electrical man , not as details as yours but they did said something was stuck and said can be fix and clean.

1st , I don't think it's because of rainy season , it's more like it's not hot.Example yesterday was fine at 50% tuner knot but today 75% only achieve the same temperature as 50%.It's 25% difference.
It's comparing yesterday and today.Just 1 day the difference it too wide.

I haven't go and check the piping but I am sure the water flow speed is same from yesterday and today and only the changes is the temperature need to set to 75% compare to yesterday 50%.
If the water flow flask is faster , I think need 99% to get 50% temperature.

What could be the reason ?