> low priority stuff to do for a bit (or a high priority short task that interrupts) seems to let my subconscious solve the issue in the background and keeps me happier and more motivated.

For a bit is ok. I had one month task, that was interrupted daily for over a year, that's how I got demotivated and burned out so much that I needed neurologist.

> two difficult tasks that were supposed to be done in parallel maybe that would be different, but I've never seen anyone asking for that.

Probably that's why you say interruptions are good, no experience of how bad can interruptions be.