You should have no problem sourcing 25mm tube as you are situated in the EU. Personally I’d use at least 3mm wall thickness if you opt for alloy for rigidity. 1.5mm would be fine for steel/stainless steel though.
Here in Australia proper 25mm metric tube is hard to come by. For some reason we are backwards and inch tube is still the industry standard for the most part.
Good luck with the build mate. Looking forward to following your progress.