OK, I don't know about those brands, so can't really comment. But I would go with "certified" cables at this point, and by certified, I mean hdmi.org certification that certify that these meet the highest hdmi spec and can carry 18 gigabytes/sec of data. For example, here's a monoprice.com cable (different sizes available) cheap, but certified as shown with the hdmi.org sticker:
https://www.monoprice.com/product?p_id= ... gJ7evD_BwEAlso, what are your Roku display settings set to? Also, check System/Advanced System Settings/Advanced Display Settings. Turn OFF Auto-adjust frame rate and perhaps set your sub-sampling to 4:2:0 instead of the default 4:2:2.
Have you set the LG for HDMI Deep Color? https://www.forbes.com/sites/kevinmurnane/2017/11/30/quick-tip-how-to-enable-hdr-on-an-lg-tv/#25ae3f716786