Have my machine for nearly 6 months now. Use it around once every 3 days. It is a Sharp ESY1019.

I see that it has a tub clean function. Is this necessary? If yes, do I need to use some specific ingredients, or can just use a simple multipurpose cleaner, or even the powder detergent?

Thanks!

Get this one.

Yes esp for those who just close the door right after washing causing mold to grow. Always open everything after every wash

get some washing machine drum cleaner from Daiso, then use it with cleaning mode
better do every at least every 3mths, even better do it monthly

I use baking powder to wash the tub, the water was very dirty after the first wash

yes, u need

Must? Nope
But you should,
Can just pour 250, 300ml bleach (for around 8kg size) and run that cycle monthly.. or buy those purposely made WM detergent. Or h2o2 stuff also can.
Else mold will grow at the back of the tube whr it's not directly visible to you and once a while, you will get those black flaky stuff on your newly washed clothes after a few years owning the machine and your clothes also smells bad