I'll say up-front I think it will be hard to beat the price for a Riverside touring 900, especially depending on where you live and what tariffs are like for you. If I pretend that decathlon was in the US and I paid the USD equivalent for the Euro price, I question if I could beat the price.
With that in mind, I would spec out the entire build--parts & prices--in a spreadsheet, before you buy a single thing. If you do this you might find that it's not worth it.
Now, I don't have any real touring experience, but I have put some serious thought into a touring/commuter build.
1) belt drive is probably off the table unless you get a custom frame.
2) If your reference is the Riverside, the most similar frame that is well-reviewed/vetted is the Seaboard GR02 (make sure you get the fork with bosses option). Seaboard also offers steel gravel/touring frames (CX01 and CX03). Darkrock is another company that seems to be focused on hardcore/remote touring, but only offers steel frames.
3) As far as groupset goes, if you want something reliable and tested, I'd say Shimano Deore-XT or CUES, and perhaps skip the hydraulic brakes for cable actuated. If you want a dropbar groupset, microshift sword looks like a good option. In any case, I have a hard time recommending any Chinese groupset brands for shifting components because they don't really offer robust options that aren't 1x.
4) for extra parts, I can reccomend ZTTO, rockbros like tripleDot has, but I also can recommend UNO for bars/stems/seatposts. I can also recommend GOLDIX, ONIRII and ZRACE as general brands. For chainrings I can recommend DECKAS, Stone, and Pass Quest.