Need help with improving core as a very flexible person

Hi amazing yoga folks! I have been lurking this sub for a while, but I would like some advice.

I have been practicing yoga for many years now, I am naturally very flexible, so that department is covered. Where i struggle is the strength, specially core. My legs, arms and shoulders are fairly strong, but my core is definitely the weakest.

I have been wanting to do a headstand and I know I need a strong core for that. Please can some help with things I can do to improve my core. I used to train mauy thai many years ago and that was when my core was at the strongest. I stopped training once the pandemic hit.

tLDR : practicing yoga for many years, great flexibility but poor core strength