@BonsaiBoiGermany
When I sprouted P. bungeana seed (lacebark pine) I gave them a nearly 4 month cold stratification. The reason I did such a long cold stratification is I wanted to plant them directly outside, in full sun outdoors. They need full sun, a windowsill is less than full sun because the glass in the window filters out or reflects away at least 30% of the light. I probably could have gotten away with a shorter dormancy, but I was not in a hurry. I do think 1 month stratification might have been too short.
Pinus bungeana is botanically in a subgroup of the subgenus Strobus. Even though it has only 3 needles. Bungeana is in genus Pinus, subgenus Strobus, section Quinquefolia, subsection Gerardianae, along with Pinus gerardiana & squamata. There are only 3 species in the subsection. The lacebark pine subsection and the strobus subsection are both in the same section, Quinquefolia or 5 needle section of the Genus Pinus. So the 3 lacebark pine species are more closely related to the 5 needle pine group that includes P. strobus, parviflora and flexilis. So biologically, it is more closely related to JWP and the other 5 needle pines than it is related to 3 needle pines of the subsection Australes, which includes Pinus rigida and radiata.
Point of that overly verbose paragraph, lacebark pine, bungeana is more closely related to JWP than it is related to other 3 needle pines like the pitch pine. Where this fact may have relevance, is the seed of P. bungeana has a fairly short viability. Stored cool and dry it looses viability rapidly after about 18 months. At 3 years one would expect seed to be less than 50% viable. Stored at less than ideal conditions, viability could decline even more rapidly.
As bonsai, the best feature of this species, Pinus bungeana, is its exfoliating bark. Key would be to create bonsai with a lot of trunk showing, so that one could see the best feature of the tree. So plan on taller, medium or larger size bonsai for this species. I am finding them in the seedling stage to be fully winter hardy in zone 5b, for others that are curious about this species.
In northern Germany, you could simply set the pot(s) with zero germinated seedlings outside for the remainder of the winter. The natural cold, with its variation over the winter, will satisfy the stratification, and they will sprout naturally in spring at the optimal temperature for rapid spring growth. Do put them under a wire mesh, cage, or a top over the pots, as birds, squirrels and rodents of all types find pine seed very tempting food.