[sausageman]

So, it's time to replace my 10 year old plasma tv with something OLED, and upgrade my rpi4 running Libreelec.

This on paper looks to be the best choice for Kodi, but I'm confused about HDR options. The LG tvs don't support HDR+ and the N2+ seems unlikely to ever support Dolby Vision. Am I looking at the wrong hardware to get the best picture when watching media files?

It's going to be tricky - Dolby Vision sources outside of real UHD Blu-ray and DRM-ed streaming services are complicated outside of demo files.  There are solutions that let you create a kind of 'pseudo DV' rip from DV UHD BDs - but AIUI they discard the second DV enhancement layer (not always present on UHD BDs) and only add the DV RPU Metadata to the HDR10 base layer (so you get an HDR10 stream with DV metadata - but not a full DV file if there was a significant enhancement layer).  This embedded metadata can improve things - but is not perfect - and the resulting file isn't completely DV-compliant (single layer DV streams shouldn't really be in HDR10 YCbCr format AIUI). Playback of this content is limited to a very small number of platforms (and only in certain containers at the moment).  It's a pretty niche area.

HDR10+ is more widely supported in Kodi platform terms AIUI - but because it's Samsung's 'sort of' rival to DV (it's not as clever as DV - but doesn't require lots of DV licensing to be paid to Dolby...) other manufacturer's like LG seem less keen to support it.  There may also be less HDR10+ content around than DV (though I think you can do HDR10, HDR10+ and DV tri-standard UHD BDs).  HDR10+ is a more open standard than DV - so is likely to be better supported in Kodi (I think some platforms already support it reasonably well) and it can be ripped I believe.

To be honest I'd concentrate on HDR10 support (and HLG in some territories) in Kodi as for many users that's 'good enough',  though you don't get 'scene-by-scene' display optimisation based on metadata - some displays kind of try to implement this themselves increasingly by analysing the incoming video directly (rather than relying on metadata).  You'll still have DV support for Netflix, Prime, Disney+ and Apple TV+ potentially (either via something like an Apple TV 4K or internal apps)

Very thorough and helpful answer, thank you