The huge negative press from the last seasons of GoT affected GRRM. You hear it in his interviews, particularly as the last season is now 3+ years in the past.

My theory is that he was really disappointed that he didn't have more control over the GoT scripts. House of the Dragon will please fans (I expect) and some pressure will come off GRRM for the rest of ASoIaF.

Just as important, GRRM of course knows how GOT ended, and I am 100% sure it is not just how Martin planned the last 1/3 of the series. He is human as well, and some time passing lets Martin get back into his own groove of how he wants the series to end.

Last but not least, if Martin plowed in and finished book 6 in 2019, most all discussion would be about is the differences between GoT season 5-6 and ASoIaF book 6, rather than just the book. Time and House of the Dragon will allow Martin to release book 6 and have it stand more on its own merits, rather than being viewed as a "Snyder Cut" of GoT.