Hi skut_estonia.
Have you and your kids tried the
Blake and Mortimer books, published in English these days by Cinebooks?
They were created by one of Hergé's other main collaborators, EP Jacobs. (Hergé gave Jacobs cameo appearances in some books; for instance he's the mummified Egyptologist "Jacobini" on the cover of
Cigars of the Pharaoh.) Jacobs worked quite closely with Hergé on
The Seven Crystal Balls, and his Blake and Mortimer books sometimes share that eerie noirish tone, as well as a lot of archaeology. They're wordier than the Tintin books - many people would say over-written! - and intentionally lack the humour of the Tintin books, and have their own style (probably not any more close to Hergé's than the Garen Ewing
Julius Chancer books you mention) but they're sort of in the same Belgian clear-line school as Tintin, with loads of beautifully drawn detail, period cars and cityscapes, etc, and action.
After Jacobs' death, the series was eventually continued by various artist-writer teams. Personally, I think most of these newer books are pretty good, some of them better than Jacobs' weaker ones (though many fans will think that opinion sacrilege!)
Cineworld have been publishing them somewhat out of order, launching their editions with what's generally regarded as Jacobs' best book,
The Yellow M. That might be a good one to try first, or maybe the
Pyramid two-parter (which precedes
The Yellow M narratively) if your kids are particularly into Egyptology.
I sympathise with your kids. I used to dream about finding more Tintin books too ( always waking up before getting to read them properly!) Mind, when I was young there were still some Tintin books (and the Jo and Zette books) yet to be translated, so occasionally on a visit to a bookshop, the dream came true!