You're funny, this sounds like a What If? from xkcd... Let's walk through this, shall we?
a) Just building a million satellites would probably involve a massive amount of CO2 output, negating any possible gains.
b) Never mind the CO2 output caused by building the required # of rockets (or obtaining the raw materials for such a massive project), and launching those rockets.
c) You can't 'set' a satellite at fixed point in space. It orbits the planet, choose orbit. Always sunny = orbit in 'equator' plane perpendicular to the sun = no shade on the earth. Other orbits = orbit partly on sunny side + partly in earth's shade. And always moving. Yeah I know about Lagrange points - doesn't help here.
d) Do the math: 1000 x 1000m = 1 km^2. So a million satellites 1...2 m^2 each works out to, ehm, 1...2 km^2. Read: 'nothing'. For example the Sahara desert alone is many thousands of km^2.
Options to improve: some mass driver, nuclear powered rockets, space elevator, building these satellites in space, using materials gathered from space as well, etc etc. Read: unproven / non-existing technology. And no we don't have time - it's running out, we need solutions that help NOW.
And we didn't get into things like space junk. Starlink has ~4000 satellites launched, aiming for 10k+, and astronomers are already complaining (rightfully, imho). You proposed a million satellites? Oookkkaaayyy...
TLDR; takeaway: do stuff on the ground, that's easier, cheaper & more effective. Like (if practical), paint the roof of your house white. Yes, that does help. :-) Plant some trees, install solar panels, commute on bicycle vs. using big honking SUV, reduce your meat consumption, etc, etc.