I had my museum completed last month but people still complain about not seeing that sly fox Redd. Let's not kid ourselves the art collection in that game is a massive grind. It took Redd over 300 visits to finish the art collection due to the random chance of spawning fakes, duplicates and limit of only buying one artwork per visit (although you can get around this by having more players on the same island).
The trick is to
After buying your piece for the day, go to your
system settings > date/time. Then fast forward to the next day. Log back in and get your mail from Redd. Delete his mail after you've received the art work (they won't delete themselves like your other mail). Save then exit the game, go back to
system settings > date/time. roll back 24 hours. Login and it'll be a new day again, and after mashing through Isabelle's monologue, you will notice that Redd's back on your island with a new set of artworks to be curated!
Rejoice! You've unlocked Redd as long as you ground hog between those two days. Don't go further than 24 hours, or the end of the week, because the NPCs reset at the end of the week. Do it until you've completed your art collection.
In Spiffing Brit's voice:
"Animal Crossing New Horizons is a fantastic game with absolutely no exploits whatsoever!"