You can find some on Amazon.

Gelfand's Algebra: https://www.amazon.com/Algebra-Israel-M-Gelfand/dp/081763677...

Gelfand's Geometry: https://www.amazon.com/Geometry-Israel-M-Gelfand/dp/10716029...

Gelfand's Trigonometry: https://www.amazon.com/Trigonometry-Gelfand-Mathematical-Sem...

israel gelfand, a top russian mathematician, has books aimed at high school students that served as education materials for distance learning in russia. the series covers algebra, functions, trigonometry, graphs, geometry, and more.

https://www.amazon.com/Functions-Graphs-Dover-Books-Mathemat...

https://www.amazon.com/Sequences-Combinations-Limits-Dover-M...

https://www.amazon.com/Geometry-Israel-M-Gelfand/dp/10716029...

https://www.amazon.com/Algebra-Israel-M-Gelfand/dp/081763677...

https://www.amazon.com/Trigonometry-I-M-Gelfand/dp/081763914...

https://www.amazon.com/Method-Coordinates-Dover-Books-Mathem...

the books by sanjoy mahajan are also a treat and teach real-world applications of mathematical and scientific thinking.

https://www.amazon.com/Street-Fighting-Mathematics-Educated-...

https://www.amazon.com/Art-Insight-Science-Engineering-Compl...

That's super weird. In eighth grade (2013-ish, not long ago) we had an entire course on geometry that was all about proving theorems and turning some given information into "the shape satisfies the definition of a parallelogram because..." The tests were exactly what you described you wanted: hour long journeys where you applied the theorems and postulates we learned up to that point to prove something about a shape. I struggled a lot but ultimately I believe I am better at thinking because of it. If you want some books:

The textbook we used: https://www.amazon.com/Geometry-Ray-C-Jurgensen/dp/039597727...

The Art of Problem Solving: https://www.amazon.com/Art-Problem-Solving-Vol-Basics/dp/097...

AoPS is pretty good but more geared toward math competitions. The reading is ok but the problems take some serious thought and if you can get your kid to do one a day s/he'll probably be better off for it.