Flooding due to rain in Bangkok

Perhaps they could build a sluice in the river before Bangkok, which would reduce the flow of river water.
Then the river level in Bangkok is somewhat lower and the water can be pumped away more easily.
At least I assume that it does not rain excessively all over the country at the same time, so that the river also has its highest level when it rains in Bangkok.

With that lock you could keep the river between upper and lower limits.

With a brother of that lock on the sea side of Bangkok you could also ensure that in a time of drought, the river water is not drained too quickly and the sea water does not easily come inland.

They don't have to be locks that close completely, but regulate the flow by making the river narrower.

Another solution would be for the wells to be cleared and/or cleaned and to continue to do so.

I'm a layman, so I'm making a layman's comment. A European with an architectural mind once published photos of the construction waste left over from the renovation of the Jomtien Beachroad promenade, with the water drainage channels full of... with the comment in Europe a contractor would go to jail for this. No wonder Beachroad floods when the drains are clogged. What about the drainage channels in the rest of the country, are they maintained and cleaned regularly? None of my business, just a comment, I don't have any problems with flooding either. I was once “caught” in a small side street of Walking Street by a super heavy rain shower, just taking shelter. At first you see that the rainwater is drained well... but after about 1 minutes all the drainage grates in the street are blocked by the plastic rubbish in the street, mainly plastic bags and rubbish from fast food chains. Once all the grates are blocked, the water rises in no time, astonishingly quickly. I was able to take some really nice action photos of it. Suppose people would pay attention to this, keeping the streets clean and maintaining/cleaning the underground drains? Just an idea... from a simple tourist/layman.

It is the rainy season and then everything floods when it rains heavily. Always has been and always will be here. Don't worry about that. What to do about it? Ha, Ha, Ha! One drank a glass, one peed and everything remained as it was. Have fun with it.

I too am only a layman in this matter, but in the absence, so far, of an expert's response, I want to ventilate my view.
Bangkok is mainly built (floats) on swamp soil. Due to the high density of buildings and the extraction of water from that soil, Bangkok is sinking 7,5 – 10 cm per year(!). And then we haven't even mentioned the rise in sea levels due to global warming.
So what the city needs are plans for (billion-dollar) infrastructural works to ensure that it is not permanently submerged in (sea) water. So far I have not heard anything about it. Apparently the economic damage is not yet great enough or, even worse, people are not aware that the economy is suffering from the annual flooding.

Global warming also ensures that the extremes (rainwater, drought and so on) occur more often, more intensely and for longer.
In my opinion, a few locks are not enough to prevent the city from being flooded by water from the north. I am thinking more of overflow areas and/or reservoirs in the upper reaches of the rivers. You kill two birds with one stone: collection of excess water in the wet season and reservoirs in the dry season.
I've read that the government is building 12 gigantic tunnels under Bangkok to quickly drain excess water. A bit of a waste of all that (drinking) water if I do say so myself. Especially when you consider that in recent years the water supply at the end of the dry season regularly necessitates rationing.
Naturally, keeping the existing water drainage free (of mainly plastic litter) is of great importance for the drainage of rainwater that has fallen on site.

The annual flooding of the streets of Bangkok may be nice for the tourists, but in my opinion it does not outweigh the (economic) damage they cause.