Advice on planting under Marri trees (Corymbia calophylla)

Hi all, I’ve been battling with planting out a little area on my property in the Perth hills for about 12 years now and all that seems to stick is a lilly pilly and some native iris.

Most stuff survives a year or two and then suddenly dies. Upon autopsy the little guys are always covered in roots from the marris. I’ve included a picture of my poor little grevillea that had been doing so well for the last 8 months. I’ve planted under plenty of eucalypt varieties before and even they’re not as aggressive as these marris.

Does anyone have any advice or native species that survive under these trees?

Thanks in advance!!