A few other interesting things in addition to your own answer:

Ray-filtered Glass Shader

Set your windows' glass shader to be fully transparent for all rays except shadows and reflections. (example)

If you use a Glass shader:

If you use the Principled Shader (not 100% sure of the accuracy on this one, would love an expert's word on it):

Comparison:

Clamping

By default, Blender have its Direct Light Clamping disabled (zero) which is often optimal, but its Indirect Light Clamping is set to 10. Increasing this value or disabling it (set to zero) helps light entering the scene, at the cost of noise and fireflies (but a good denoising usually does the trick for that).