As a beginner, I found this collection of simple millis() explanatorials very useful https://www.baldengineer.com/arduino-millis-examples.html to allow one’s microcontroller to do other things while FastLEDing.
I recently developed a library for making this kind of coding a lot easier. It gives you the illusion of concurrency by hiding all of the millis() timing junk inside macros. I’ve written a decent amount of documentation and examples. You might find it useful: