Ciao Adriano,
As Ty said, I think you are right on the money.
Going with ambisonic ambiences can ease some of the burden of creating a basic environment.
You can then add, layer, pan and mix foley and additional sonic elements to enrich and better detail your ambiences.
Back in the day, not having any 5.1 or Ambisonic sound library, I had to build my ambience track layering different stereo elements from SFX Cd libraries (Sound Ideas, The Hollywood Edge, etc.), or DAT recordings, and then duplicate all these elements and shift their timing in order to have different events hitting the surrounds…
I still use this technique today considering the large amount of SFX that have been accumulated at the facilities I work with.
Hope this helps.
Cheers,
Luca