I like the simple mechanics of the game a lot. But I do think that sustaining stress even when successful has the effect that doing anything is always risky, leading to players behaving conservatively. Which led to me as the GM hold back on negative dice, even when they would have made sense for the narrative. For a fun game you want people to be trying lots of things out, not be scared to try things because they might get hurt.
To make it more lenient, how about adding a rule that each un-cancelled success also removes a negative dice. So negative and positive dice with the same number are removed as usual, but also, any successes also cancel a negative roll.
So say I'm attacking a zombie with dexterity 3. I have a baseball bat and attacking it from behind (plus two positive dice for three total), but I have a damaged arm from a previous trauma, it's raining so I have loose footing, and the zombie is horrifying (3 negative dice).
Say my positive rolls are 1, 3, and 5, and my negative rolls are 4, 5 and 6. Normally I would hit the zombie for two damage, but also sustain 2 stress myself! But if successes also remove a negative dice, then I would hit for 2 damage and not take any stress.
Or imagine my positive rolls were 1, 4 and 5, and negatives 2, 3, and 4. I would hit the zombie for one damage, after the 4s cancel there are two remaining negative dice, and my success would cancel one of these, but the remaining negative dice would cause me one stress.
I think this might be nicer because successes will actually feel a bit more like successes.
The only problem is that this might get too easy for the players once the resistance kicks in from the stress tracks. So the resistance rules could be dropped entirely. They don't quite make sense to me anyway. If you have some physical stress like a twisted ankle, you would think it would make you more vulnerable to additional physical stress not less! Like, when has a twisted ankle made you suddenly more resistant to other injuries? Seems strange to me.