Slash and burn: Grim Reaper approved!
As Wiki mentions, slash-and-burn becomes a tool for already established agricultural societies when they need more land. As you can see in the Wiki photo, the trees seem relatively thin, the stumps suggest trunks less than 6 inches thick and most of the charred remains seem much thinner than that. A forest fire in lush virgin woods, unless it’s thorough and big, is going to leave an even less passable jumble of giant charred logs. Plus, again, in a area with a decent amount of rain, getting the fire going would be difficult.
IIRC the Pacific Coast indians would get logs for dugout canoes by packing a ring of clay around the trunk, loading it with moss and setting it on fire. The tree was too moist to burn, but it charred a few inches deep so they could bash that away with stone tools; rinse and repeat, run away when it falls. That works for a tree when your obbjective is lumber, but not terribly useful for clearing, especially as it leaves stumps.
Yes, the Iroquois for example used slash and burn too, but apparently they rotated fields on a regular basis. They were semi-nomadic and relocated every few years to let the fields recover (by regrowing forest). I suspect they returned to a location after a decade or so, the trees would be still mainly brush not 2-foot-thick trunks. Plus, it was miles to the next village so a runaway forest fire was not as big a risk. And again, they adopted this technique and the crops after learning of agriculture from more southern, plains-area tribes.
Thick forest was an obstacle to many civilizations well into the iron age. Small village clearings in the forest tended to be the best model, not condusive to large centralized city-states.
I agree with what you are saying here, but I do not think you can pin the lack of large, civilized societies along the Danube to thick forest cover. I have no doubt that these forests existed, but that could not be the sole cause for the lack of early cities there. For example, the early meso-American cultures appeared to carve their great cities out of the forest (and when abandoned, the forests reclaimed them).
Rather, I think the lack of suitable native plants that could be harvested for [enough] food is the primary culprit. Example - In the Tigris valley, and other areas, native wheat strains produced larger and more seeds than varieties in other areas (maybe owing to the more mild climate). Once people figured out that this plant could be grown close to their homes, and as long as they stayed put year-round to tend the crop (and others), rather than wandering around searching for food, you have the makings of early cultures: The local wheat, when cultivated, produced a surplus, which meant people did not have to worry about finding enough to eat each day, which led to extra time for inventing, crafting, building, organizing, spiritual thinking, more kids that survived, social strata, leaders, rules and laws, armies, etc. that are all hallmarks of what we may call “civilization”.
The Danube simply did not have these advantageous local plants, so people living there “civilized” at a slower trajectory until better crops, technology, etc. arrived through trade. That is not to say they were not “civilized” to some extent, but they were not as successful until contact with the more successful areas further south.