You can get that setup from aquarium shops along Jln Sg Buloh. About RM120. Fibre glass chamber that sits on your pond. Pump is submerged in pond. Pump water up, go through filter media, flow back into pond

Submersible pump needs to be immersed in water, because they can get very hot. No water to cool, very fast burn.

I dun like the idea either, because with inferior pumps, there can be current leakage, and get give your a nasty shock if you are directly connected to the earth e.g. wearing wet slippers.

You can try this type of pump-filter for small ponds. Still can leak current, but less likely as most part are not in contact with the water. Gotta clean more often though, due to less filtration area. Or set up secondary filter chamber outside.
Have to find somewhere to hide it and the long hoses, though.

If small size pond really dun have to worry too much. Once the ecology has settled in, it takes care of itself with little maintenance.
I only need to every week (or so) remove extra water plants (tend to overgrow) and change 1/3 of the water and add water conditioner. Every 3 months wash filter.

The key is (i) do not overload with too many fish (ii) do not overfeed

In fact, my best performing small pond, the main filter is just a a tilted ornamental vase fill with bioballs and an activated carbon pack. Water is pump in from bottom and overflow from the top to create a waterfall effect for oxygenation. The three medium size koi inside there are the happiest and the most playful.

I am looking forward to set up two 4 x 6 fibre ponds in my new house. (one for me, with big macho fish; the other for wifey and her cutesy guppies)

But do be careful with the pumps. Buy good ones and change them every year.

For small pond with little surface area, i will do 24/7. Only off during clening and maintence. The most important function of the pump is to provide aeration, either thru bubbling or the waterfall effect.