There is a lot around the Cheviots. I’m not the best person to point out where but around Wooler for route planning purposes.
There is a fair bit in the borders as well but none of it marked an may be quite hard to find on your own if only going once and pushed for time.
Kielder can be good if you like the riding it offers. It is getting more like Glentress in terms of what it offers but it is more about big XC rides in wide open country in my experience compared to GT being more about berms jumps and drops. I love Kielder – riding the Lonesome pine trail then going over the border to Newcastleton is one of my favourite rides – but not everyone likes it. Also it is exposed in lots of places so a bit grim in the wet/wind.