Another illustrator from the 70's was Ingahild Grathmer[1] which was said to be a favourite by Tolkien himself[2]. Maybe he was polite because of the noteriaty (not sure if known at the time) but I do like them as well. Have a look at the documentary on YouTube:
https://youtu.be/rNqVqzIxi3A&t=24m19s
Tolkien was generally favorable to any illustrations that followed the text; he actually explicitly mentioned that he was creating a “legendarium” which would have different interpretations.
What he didn’t like was illustrations by those who never even read the descriptions, or created things based on other media (Tolkien dwarves vs Snow White dwarfs, for example).
(Go to 24:19 for Ingahild herself)
[1] a.k.a. Margrethe Alexandrine Þórhildur Ingrid (https://tolkiengateway.net/wiki/Margrethe_II_of_Denmark) [2] https://www.mentalfloss.com/article/66764/time-queen-denmark...