Ok, let me report back later with the correctly applied advancedsettings.xml file in play as it could be as simple as that… hopefully!
Ok @sam_nazarko, it turns out the tweaked advancedsettings.xml I added fixed this buffering issue for h265 .mkv files, but the h264 .mkv files still struggled. I can live with that as I assume the bitrate is significantly higher on the h264 files to play 4K UHD files.
The default advancedsettings.xml in OSMC didn’t work for me, as there was not enough overhead to cope with the the 4K content in h265 format.