Studio Lens Question

I am a hobby photographer who mostly takes photos of my kid/family and candids about town/travel. I have an A7C2 and 3 prime lenses (Sony 35mm 1.8, Sony 50mm 1.8, Viltrox 20mm 2.8). I've recently gotten more and more interested in dramatic studio style portraits/lighting and I am slowly building up my lighting gear. I'm not looking to start a business by any means, but I get joy from being able to tell a family member or friend "Oh I can take that photo for you." (e.g. My mom recently started a new job in an executive role and I rented a backdrop and an 85mm prime so we could update her woefully old and frankly poor DIY headshot).

So my question is, what would be an ideal lens for this next phase? Should I look for a zoom that gets around 135mm for more dramatic flattening or look for a tried and true ca20-ca70 zoom? I'm not looking for the baddest, fastest lens out there, but one that will do just fine.