Yes, painting the light well white does help a bit, and will be better at bouncing light down if this becomes more than a paper exercise. (You haven't said if this is a serious prospect of being built, or just for fun and daydreaming.)
Blender is completely free, but the downside is that it is very powerful and a lot to learn and understand. Not so good for iterating over your designs lots and lots like SH3D is.
As to your laptop. I bought mine (Dell XPS 15) in April 2017, although I did kit it out quite well (Quad Core, NVIDIA 1050, 32 Gb RAM, 1 Tb SSD). The image I posted was rendered using Blender on that machine. I don't know how that compares to your kit, but you might not be as restricted by the hardware as you think. I don't remember timings, but maybe 10 or 20 minutes for a frame like that with the CPU. (I don't use the GPU for "reasons"
)@bdfd The OP did mention skylight shafts, but mentioned being unable to figure out how to create them in SH3D. I did it on my model, but that is of simple pitched (i.e. /\ ) construction, so there is a way to insert windows into very tall walls, then export, and re-import while rotating to make the walls into the roof.
My physical house has both roof lights and what bdfd called skylights. I've only heard those called sun or solar tunnels.
Roof lights are great for getting huge amounts of light into an area, but you have to consider the issues with opening (ventilation) shading (sleeping) and access (cleaning and maintenance). Most competent brands offer powered (even solar powered) shutters, blinds, and opening mechanisms to solve most of these.
Sun tunnels are what I would call the last resort measure. Yes they get a surprising amount of light into a dark space, and they can pass through quite deep loft spaces while doing it. Here are the negatives: There's no view out, so it makes a room feel a bit like a cell. No alternative for egress in an emergency. I think the external dome is rather ugly, although some brands have a flat roof collector. They are questionable from the thermal insulation POV.
