Bake the light, keep the frame rate.

A lightmap is a texture that stores the light that falls on each point of a static surface, bounces included. Make the second UV map in Blender, export the FBX for Unity, bake with the Progressive Lightmapper, choose Baked or Mixed lights, and light moving objects with Light Probes and Reflection Probes.

STAGE

Scene

LMB orbit · Wheel zoom · MMB pan

Lightmap

LMBOrbitSpacePlayCtrlZUndo

KEY IDEAS

Light that costs nothing at run time

Global illumination is too slow to compute every frame. Baking computes it once, in the editor, and stores it in textures (lightmaps) and in points in space (probes).

Lightmap UVs: A second UV map (UV1 in Unity) with no overlaps, islands in proportion to their real size and a margin between them.
Lightmap Resolution: Texels per unit (metre) of surface: a texel density for light. Scale In Lightmap changes it per object.
Bounces and samples: Bounces carry light into shadows and corners. More samples, less noise; a denoiser removes the rest.
Baked · Mixed · Realtime: Baked lights exist only in the bake. Mixed lights bake the bounce and draw direct light in real time. Realtime lights bake nothing.
Static: Only objects with Contribute Global Illumination take part. They receive the bake from lightmaps or from probes.
Probes: Light Probes give baked light to moving objects; Reflection Probes give reflections of the real room instead of the sky.

BLENDER → UNITY 6 (URP)

The workflow

The names are the ones of Blender and of Unity 6 with the Universal Render Pipeline.

Blender: apply the scale (Ctrl A), add a second map in Object Data › UV Maps, make it with UV › Lightmap Pack or Smart UV Project with a margin. Keep the texture map first.
FBX export: Selected Objects, Mesh, Apply Scalings FBX Units Scale, -Z Forward, Y Up, Apply Modifiers, Smoothing Face.
Unity import (Model tab): Scale Factor 1, Convert Units, Bake Axis Conversion; Generate Lightmap UVs only if you did not make your own.
Mark the static objects (Contribute Global Illumination), set the lights to Baked or Mixed, open Window › Rendering › Lighting and press Generate Lighting.
Moving objects: add a Light Probe Group, or set the URP Asset's Light Probe System to Adaptive Probe Volumes and add a Probe Volume.
Shiny materials: add a Reflection Probe per room, with Box Projection and its box fitted to the room.