[noggin]

.Now I have Lumix GH5 and it is considered the best camera in the world for video but there isn't UHD HDR10 Rec 2020. I don't know what to do. ufff

I thought the GH5 had an update enabling HEVC recording to UHD HLG rec.2020   

I know only 25p and not 50p.

If I shoot in rec.709 with my camera and I like to watch the video with Shield TV, is it better to export with BT.709 or BT.2020? 

It depends on your TV's handling of SDR Rec 2020.  You are simply moving the SDR Rec 709->SDR Rec 2020 conversion to Edius/Resolve etc.  (Your TV will then convert Rec 2020 to its primaries)

If you have your Shield TV configured for Rec 2020 output and export in SDR Rec 709 then the Rec 709->SDR Rec 2020 conversion happens in the Shield TV.  (Your TV will then convert Rec 2020 to its primaries)
If you have your Shield TV configured for Rec 709 output and export in SDR Rec 709 then the Rec 709->your TV's primaries.

SDR Rec 2020 isn't a widespread format.  How TV's handle it can vary.  (SDR in general can be handled as 2.2 or 2.4 power-law gamma, BT1886 EOTF etc...)

Out of interest - does your TV have an internal media player?  What happens if you play your Rec 709 UHD files in that rather than on the Shield TV?