I just found a solution that works pretty well: check the outlet sections of chains you already know, like El Corte Inglés, Carrefour or hypermarkets. I'm not talking about clothes from three seasons ago, but brands like Dockers, Dickies or even some of those stores' own lines that just get less promotion. The difference is huge compared to Shein: the fabric actually holds up through washes, the seams don't fall apart after a week, and in the end you're paying between 40 and 60 euros for pants that last you years.
Also look into Spanish or European workwear and technical clothing brands that sell to regular people. Places like Decathlon have some pretty decent cargo pants because they're designed to handle movement, not just look good in a photo. The thing with those clothes is they prioritize durability because a lot of their customers need them for work. They cost more than fast fashion but less than premium brands, and honestly the fabric breathes different, the pockets stay in shape.
What I'd do is just skip platforms where they sell you a hundred variants of the same product for 12 euros. That money you save disappears when you find a hole in the thigh or a zipper that gets stuck. Better to spend a bit more but buy something that won't bail on you in three months.