[INFO] which CRF...

[Selur]

so how can we explain the big differences ?

The effect crf strongly depends on a lot of other settings, so unless you compare all the settings just looking at the crf and the file size doesn't say anything.
You can use MediaInfo to see the settings that are used in case x264 or x265 were used by ffmpeg/handbrake/Hybrid.

what are your advices (parameters) to encode in H265 with Hybrid please ?

1. I usually encode to H.265 with x265 and a crf of around 16 (or even lower), but that will probably produce a way larger file size than you get.
2. When reencoding I usually make sure to do some source adjusted filtering to remove existing compression artifacts which were introduced by previous compression steps.

Cu Selur

Ps.: you probably have more luck getting compression settings recommendations for such things over at https://forum.videohelp.com/