One quick question without reading every post , have you checked the flush valve is not funny open ? I know that really simple question but just checking
I'm jumping on this is a solution.
So we know input tds is 160. If the water is coming out of the pure at a tds of 4 before it goes to DI, the r/o is working at 97.5% efficiency so their isn't a problem there. The r/o should work at between 97 and 99% efficiency.
"I think the water is flowing ok as there is fair rate come out of the drain pipe." This indicates that too much water going to waste and why I agree with Ian's post.
With your r/o running, measure how much water is going to waste (use a 2 litre plastic milk bottle) over say a minute, and then do exactly the same for the pure, also over a minute. If you have a higher waste to pure ratio than 3 waste to 1 pure, then you need to adjust your waste valve to get the right ratio. Some manufacturers use a small insert that fits into the waste which restricts the waste and forces more water through the membranes, while others use a tap that you have to adjust yourself.