Thank you for your answer, nemophoto!
In the meantime, I have found a vendor in Germany whom I will try: TKexe.de. His software allows everything I need, the calendar can be made in "all" languages, the max size is A2 (41x61 cm) for now, at my request he has promised to add 61x61 cm square and alert me by e-mail when it's there. The prints of this size are made on a Canon imagePROGRAF iPF 6100 with 12 colors, 1200 dpi - looks like yours, nemophoto.
The price for an A2 calendar (portrait or landscape) is 53.30 € ex shipping, incl 19% german VAT. Sizes up to this are printed on a Fujifilm Frontier 550. - So if he can offer A2 in landscape orientation, he must have access to spiral binding 61 cm.
No I have not thought of printing myself. Landscape photos must be big in my view, and a large enough printer is out of my reach.
Concerning my question 2 from my original post: I can now see that the question is nonsense. Upressing means resampling the image at lower resolution; that IS lowering the Quality Factor.
Good light! - Hening