How to multitask with interrupts on Arduino?
There is no real multitask on Arduinos. The Arduino can only do one thing at a time.

But, you can do things sequentially, one after another, so fast, that a human will think, that it happens simultaneously. This involves a non-blocking coding style, which needs no interrupts


Every delay in your code (at least the long onces) need to be removed. if you want to do timed things, you should use the coding principle from the BlinkWithoutDelay example, that comes with the Arduino IDE.
Also no function should wait for something to happen. Think of a function, that waits for a command from the serial interface. Blocking code would wait indefinitely, until a command was received, preventing any other code from execute. Instead the function could exit fast, when there is currently no command to process, leaving time for the other code to execute.


The whole principle relies on the loop() function executing very fast, so that your code can still be reactive. This works for most of the situations. Only sometimes you need to react that fast, that you need interrupts. Your usecase doesn't seem to belong to the interrupt needed faction.