Hello everyone, after doing some even more 'scientific' tests I've got some results.
They match with others on this forum, but they definitely disagree with the great video we all saw suggesting that sharpness +1 is the 'natural' (no processing) setting.
First of all the tests I'm doing, like in my first post video, are to put encoder to the extreme situation: contrasty scene, moving nearby small fine details (greenery moved by wind) and on same imagine a more far (but not too much) equally detailed background (loads of trees) in the shadows nearby a lit sky. In this demanding situation, using sharpness +1 underlines the work behind the compression and the GOP frames are the 'sharpened' ones while the others are very compressed, and that leads to all 'flickering' problems (details, edges, shadows/lights) we see everywhere on this forum (and that I found more disappointing than I expected).
To be honest all the nice Youtube footage I've seen, I realise now, is filmed from very high in the sky, in stable light, and so you get a frame without much fine detail, without moving lights, and all frame moving more uniformly and slowly (not bottom half moving 'fast'), or other are test shots on a bunch of trees or 'static' scenes with drone not moving. Now that I know that defect, I spotted the same 'shadows/light' and details flickering in loads of these, and yesterday I even spotted it clearly on a recent BBC documentary footage that was filmed by a drone!
I think the problem here is that the small sensor cannot (understandably) get too much detail, and DJI try with sharpness to save situation a bit, tuning it with the classic shots everyone will do (high in the sky), but it shows the compression limits on any more 'demanding' situation.
Someone else (many) here said that the ideal settings for sharpness was -2, and I have to say it's actually true. I found that sharpness -2 produces the most 'natural' result, considering that is much better to actually try and sharpen a bit in production, where you have much more control and you can actually sharpen more uniformely ALL frames not just the key ones (and so no flickering).
I tried all sharpness settings in both 4k and 2k at 30fps, and interesting is also that 0 in 2k has same 'defects' of -1 in 4k, -1 in 2k has same 'defects' of -2 in 4k and so on, really almost identical, but once you arrive at -3 on 4k (and so my ideal -2 in 2k) you get a much worst result in 4k than in 2k. Someone on this forum said -2 -1 -1 2k 30fps is the ticket, indeed I found the same. I've done all tests on the same field and same flight path. I'll probably use 2k -1 sharpness when doing less demanding footage in terms of details/movement/light, but the sweet spot is for me -2.
To be honest working on this I started realising that anything sharpness 0 and up looks to me a lot 'digitally' processed..it has that 'wow' factor at first but looks more 'cool' than natural/film, is a lot like all those HDR pictures...is cool and has wow factor to the unaided eye, but it looks very unnatural and 'fake' to me, as I like more 'real' photo or video.
Below the same place of my first video filmed in 2k -2 -1 -1 30fps, conformed 24fps and graded and sharpened a bit to be still natural and 'film' like (to my taste), the sky is blown out as I was filming in Natural (I'll be back in DLOG as it's not the problem and has a wider dynamic), but in the end I wanted sky very bright to be in contrast with the critical shadow area of the trees:
