Present Continuous vs Present Simple – Difference Between ‘I Still Work’ and ‘I’m Still Working’

When native speakers say

I'm always {doing something}....

it's as if they're gently berating themselves that they persist in making the same error or slip-up.

I'm always misplacing my car keys!

I'm always forgetting my wallet!

I'm always mistaking him for his older brother!

Those are things you find yourself doing again and again. Such statements are an exclamation, more or less.

You can express a similar idea with simple present.

I always misplace my car keys!

but for many speakers the simple present wouldn't be as gentle and forgiving as the continuous, and there would be a little tinge of angry frustration in the statement.

What would NOT be idiomatic is to use the present continuous to refer to your standard (volitional/intentional) practice:

I am always eating lunch at that restaurant because it's close to my office. ^{not idiomatic}

Ask yourself what is the difference between "I fish" and "I'm fishing". One is a general, blanket statement; the other refers to a specific ongoing event.

Likewise, "I refuse to work Sundays" would be a blanket refusal - you won't work Sundays, any Sunday. "I'm refusing to work Sundays" sounds like you are currently refusing to do so. This could imply that it is a temporary situation, or that you are engaged in some currently ongoing argument over the matter.

Of course, your specific example says "this Sunday", which limits the reach of either statement. Still, the former sounds more like an immoveable stance, whereas "I'm refusing" sounds like an ongoing discussion which could be subject to change.