If baking lightmaps takes too long, it's likely because the texel density is too low, which means it's attempting to bake a huge texture. The default texel size (0.1) is tuned for small scenes and is probably too detailed if you're only baking indirect lighting (where high-frequency data is uncommon). Increase the Lightmap Texel Size property on your imported 3D scenes in the Import dock, then try to bake lightmaps again.
Also, it's recommended to use the Low quality preset and disable denoising at first to speed up initial baking. Then, once you're happy with the lighting setup, increase the quality and enable denoising.
As for GIProbe, it's generally not a good idea to use a single GIProbe for an entire scene. Instead, you should use multiple GIProbe nodes with lower subdivision levels to cover individual buildings or areas. GIProbes can blend with each other in master (and maybe in 3.x too). Higher subdivision levels require a lot of VRAM and are very slow in 3.x, but they're significantly faster in master. They will still require a lot of VRAM though – in a scene with complex textures, you'll want a GPU with at least 6 GB of VRAM to render a GIProbe with 512 subdivisions smoothly.
