Interesting post. We have a 2018 Highlander. I dumped the factory fill around 2,000 miles, and ran that new fill on up to the 5,000 mile car mark, making the 2nd run about 3,000 miles. No issues. Have been using Amsoil XL0w20, with Amsoil filters. Since then, I've gone 5,000 miles on the same mentioned oil/filter setup ... no issues, and no consumption. I'm going to stick to the 5,000 mile interval, I think.
I'll note ... when you use the owners manual factory specified fill capacity of oil ... it still registers low on the dipstick. Ours takes 6.1qts to bring it exactly to the "full" mark on the dipstick. Do you note the same on your 2019?
One other thought to leave you with ... I changed the rear differential fluid out at 10,000 miles ... and was horrified at how black it was, and how much "fuzzy" material was on the magnet! It only holds about a quart & is an easy job ... I'd suggest you look at yours as well.
Am interested in your input here.... thanks.