my hypothesis was proven true: the main bottleneck of a minecraft-like is in uploading chunk vertex buffers to the GPU, not in generating the meshes themselves
in Vulkan, after mapping your vertex buffers to host-visible memory, you could create command buffers for each which signal fences to tell the game they're ready to draw with, and submit them on every other queue not being used for rendering. on my GPU that'd be possibly 18 parallel queues just for uploads 👀
with modern OpenGL you might be able to use mapped buffers and fence objects to manually double-buffer chunk vertex buffers, but it would heavily depend on driver support