Which tree would you suggest?

Probably not spelt right but Stenophyllia rings a bell. I had to chop it all down this year to paint the fence so all the effort has gone into new growth. It's supposed to zig-zag to and fro but i think i feed it too much Seaweed in order to gain height and block the neighbours out lol.

 

Indeed, you are quite right.

... and I've bought Bamboos that are 2M and in 10-15L pots, but I think they would have struggled in those sized pots if I had left them there. But I ain't tried it.

The O/P wants a 2M plant in a 50cm pot ... I have bought decent sized Bamboos on eBay that have outgrown people's containers, but they have been in bigger pots than that. I also see Bamboos listed on eBay that have clearly outgrown [what seem to me] very small pots, and they look weedy as a result. But I haven't tried it, so with adequate water and feed they may be fine.

Other thing to be aware of with a small pot / tall plant is the blow-over risk. Worth anchoring the pot somehow if the "canopy" will make it top heavy. My small urns on a balustrade here are bolted down with threaded-rod - presumably the previous owner lost the first batch - which would have been an expensive mistake!

I think I read that the Zig-zagging of bamboos is caused by fast growth - it is growing so fast that one side is faster than the other. Presumably the plant then corrects (geotropism or whatever its called) to try to be vertical for the next "node". So for those varieties that do that (Phyllostachys aureosulcata?) maybe they need ideal heat and moisture to be at their maximum growth rate?

 

if you want something that looks as good as bamboo for a pot, look at miscanthus

they come in all forms and sizes

 

Great idea Dim, I think they would fare much better in pots (than Boo's). Those Zebrinus look nice my personal favourite is Molinia Caerulea "Transparent" which I think is lovely to look through/over when it is fully grown in, say, August-November