For a good "parts erupting" effect, you need the broken edges of the armor to surround the erupting nodule. Take a look at Death Guard models for inspirations.

Normally, the bent/broken armor is done in the initial build phase by literally breaking pieces and bending the plastic (heat them up s little first when bending). As an add-on here? You could sculpt it out of green stuff.

Make a 2mm thick ribbon of green stuff roughly 8mm wide. Cut a jagged edge down the middle to make two strips. Blend the smooth edge into the existing armor plate while having the jagged edge rest on your model's bulbous flesh and tendrils. If you can, curl the jagged edge away from the growth.

Would be cool if there were spots that metal is bulging pre-burst

I think tumors would take it more over into Nurgle territory (as your MidWinter tutorial reference is). I like a lot where you’ve already gone with the tentacles and other details.

If you want some more chaos greeble, perhaps airsoft BBs into masses of greenstuff, with the BBs painted as eyes.

Something as simple as shading the border between the mutation and the armor with agrax earthshade would make it pop better. The armor and mutation being similar shades of blue make it blend together. Marking the border with more distinct shading would help I think.

If that thing don’t got eyes how’s it gonna see