Drainage of the terrace - How should it be done?

Yes, the soil here is also clay and it drains very poorly. So better the drainage after all. Let's see if I can even still reach the downpipes...

Has anyone of you ever made a substructure for a terrace out of aluminum? Sounds somehow tempting, since presumably after 15 years you would only have to replace the top boards.

We are wondering:

- which profiles can withstand how much load and what span can be bridged with them?
- Also, we are unsure if the metal might possibly shine through the 2mm gaps between the boards?
- Will it possibly squeak after 2 years?
- Would something like that be affordable?

Anyone got a tip?