Iirc it wasn’t 10-12 episodes worth of plot in 6, but rather 40 episodes into 13. The original plan was to have 10 seasons, 10 episodes each. Then around the end of s6, the showrunners were in talks with Disney to work on their own Star Wars trilogy after GoT ended. Their solution was to cram the rest of the plot (40 eps across seasons 7-10) into 2 seasons, which ended up being 7 eps for s7 and 6 eps for s8. After this royal fuckup tanked the brand, Disney refused to hire them