25mm conduit is not ready available in hardware stores in DK nor GE. I don’t think it is used very much in DK. Some online hardware stores have some that they 25mm/1inch. It is not clear whether this means 25.4mm or actually 25mm. I would not surprise me if both type exits. I found some 25mm tubing used for clothing racks in dimensions that fits to my needs. I dont even need to cut anything. The down side is that it is only 1mm think, but it will have to do for now.
25mm railing tube is also available but 30USD pr m it is a bit expensive but worth considering.
You have a good point regarind plumbing vs. conduit.
I think it is great that you are also making a 25mm IE version and appreciate the extra work.
I did not expect you to make the parametric version, just throwing the idea out there.