Stuka’s incendiary barrage is underrated af. Not sure why you want to replace it if you’ve never used it.
I rarely used it because I think it comes quite late. The times I used it it was alright, but honestly a direct hit with the normal barrage would probably be more effective. It's not a Katy rocket that kills a model and deals AoE, one of the StuKa rockets can kill 4-5 models if it hits right. I just think the advantage is maybe not worth it for the mun cost. Then again I'm not that experienced with it, so if you have further insights feel free to share.
I mean i don't know how strong they'd be but if you WP barrage and then immediatly HE barrage that could be deadly in its own right.
Not immediately. As I said, shorter CD than the normal barrage. Normal barrage is ~2 min, so maybe 1 min would be appropriate? This could also allow for a completely different use of the units if you want to invest munis in it.