i want to tell what i am currently doing. maybe it is helpful to others:
for now i have finished my 200+ of hours trying to make practical approaches to GI with skyboxes and light probes and reflection probes, and have decided that its to slow (cpu cost is horrible as nearly nothing which is needed for realtime processing of light probes can be moved to other threads, from Unity - Scripting API: SphericalHarmonicsL2 to Texture2D). so even if you raycast quickly on cpu with Unity - Scripting API: RaycastCommand these days, you will not benefit from it as it requires you to build strange architectures (to get the raycasting async) while the most work will still with SphericalHarmonicsL2.Evaluate() and SphericalHarmonicsL2.AddDirectionalLight() and those cant be moved away from mainthread. atleast AFAIK, if someone has better proposals, i am happy to hear.
so these GI steps remain as realistic paths in summer 2022:
either use unity free lightamp based gi (i dont want to, as the bake steps are terribly annoying slow for me in my projects, even i have a very good cpu and gpu).
ENGLIGHTEN The Courtyard | Tutorial Projects | Unity Asset Store
or go with a full scenery GI solution: i am using voxelization
LUMINA LUMINA GI: Real-Time Voxel Global Illumination | Particles/Effects | Unity Asset Store
or use a SSGI solution: maybe you are contempt with a screen spaced gi effect, there are enough available on github and the asset store for built-in + urp + hdrp plus also unity offers free solutions on newest LTS(es).
H-TRACE H-Trace: Global Illumination and Occlusion [ RELEASED ] SSGI with nice reflection probe fallback
RADIANT Radiant Global Illumination | Fullscreen & Camera Effects | Unity Asset Store
SUNFALL Sunfall Global Illumination (SSGI) | Fullscreen & Camera Effects | Unity Asset Store
or go with gi “tricks”
UPGEN UPGEN Lighting Standard | Fullscreen & Camera Effects | Unity Asset Store