Use of existing pavement base

... will try it in any case - would you have a tip on how to maybe still get the project done "cleanly"? Many thanks!

No, because that simply can hardly work that way - I mean, to train paths with it.

You can try, maybe it will look decent enough for your purposes.

I do find the pattern interesting...

We also really like the style and above all it is cost-effective ... I will definitely present the final result - I hope that it will then be visually appealing and, above all, sustainable to walk on ;-)

I also think the idea is good. However, off the top of my head, I really wouldn't know how to lay it properly. You obviously have to make sure overall that your current substructure has a height difference of the stones plus 2-3 cm. Also, sand will settle once it has been loosened. So you have to set the flatter stones a bit higher (1-2 cm) than the higher stones, as these will give way more when compacted. It's worth a try, but I hope for you that you don't want to pave 100m² like that, as it could quickly become a lifelong task.

If the ground has been like this for 30 years, the stones will never settle to a roughly even height.
You will always have tripping hazards in it.

And if a postman ever trips over it and breaks a leg...

I also once paved the access path in the front yard myself with small granite cobblestones 4/6 under the same conditions as you, so I do not conform to the previous speakers.

We removed the concrete slabs and loosely indicated an edging with 10 x 10 pavers. I paved a curved path, so I could later align the edges and was not dependent on cutting tools. Then I repeatedly threw gravel onto the solid base so that I had about 3 cm working depth for the stones. With a rubber mallet, I knocked the small irregular stones into the working sand and arranged them as semicircles. Since the granite cobblestones have very individually curved surfaces, there were no visible major mistakes. On small areas, you also repeatedly check with wooden blocks whether it is straight in itself. It is tedious. Without a doubt. I can also imagine that a paving stone of, for example, 10 x 20, which has to be individually embedded next to an already defined height, is more difficult than a granite stone of 4 x 6. With these small stones, you simply keep pushing sand underneath, preferably more rather than less, and knock it into the surface so that the excess sand is pressed out. This, as already said, is not or only very difficult possible with a larger clinker stone. Basically, you have to take the lowest stone as a basis and height measure. All other stones need additional gravel/sand to reach the height. In doing so, you will basically end up higher than the former pavement with the precast concrete slabs. These are 5 cm thick; your paving stone adds twice as much height with 10 cm width. For the main path you should pay attention[B], if you do not work precisely, that there is a risk of accidents for those who might have to use this path![/B] I would not worry about frost. But it may be that your red clinker bricks will effloresce due to moisture and then will no longer look so nice.

Nowadays, I would only implement this idea on a side path and not on a main path.

However: I, too, would start with it first and see if it is possible on half a meter.

Have fun puzzling :)