I don't read the books, I mostly read the wikis. But I hink this was bound to happen regardless of whether D&D had the last two books or not. Rewatching the earlier seasons has made it clear they never understood the books or even liked some of the characters. They were playing favorites from day one, and even when they had material to adapt they'd ignore it if it didn't suit the narrative they wanted. It's why Cersei has always been Carol the Soccer Mom, Jaime got shafted after season 3, Tyrion was white washed, Jon became a moron (even as early as season 2), Robb broke his vow for love not honor...
There was plenty of material (almost 2000 pages worth) they could've adapted for season 5, but they weren't interested in very much of it. Key plotlines like Dorne, the Ironborn and Stannis' war for the North were all altered significantly, and fAegon was excised entirely. All of them will feature heavily in Winds of Winter, and whatever is planned for Dream will be the fallout of those narratives. Given that these storylines don't exist in the show, there was always going to be a need for original material.
The later seasons may have benefited from a clear roadmap of how Martin was going to handle everything, but I'm not convinced it would've made much difference. The cast of Game of Thrones in season 8 would not behave the same as their book counterparts, and so many little things have been changed that they're effectively different characters. There would've been no way to map the storylines of Winds & Dream onto GoT by that point. Benioff and Weiss were only ever interested in the core cast they retained over the years, and the outline Martin provided them was all they really needed. The ending still would've been rushed because D&D wanted out, not because they didn't have the books.