Hi u/cogentat - given that the $1,599 Cinematics Sigma 18-35mm T2 is a rehoused still camera lens and not a purpose-built parfocal cine zoom, you might want to consider spending $690 more for a DZOFILM Pictor 20-55mm T2.8.

This is an actual parfocal cine zoom lens that will hold focus as you change focal lengths.

Here it is on the BMPCC 6K:

• BTS Angel Photoshoot ft. BMPCC 6K & DZOFILM 20-55mm T2.8 Pictor Zoom by Duke Pham (4K)

• Dzofilm Pictor 20-55mm t2.8 test footage by Leon Constantine (4K)

• BMPCC6K + DZOFILM 20-55mm T28 Test Footage by Duke Pham (4K)

And on the URSA:

• Angelic Jayde - DZOFilm Pictor Lens Test by Tim Butt 2 Productions (4K)

• The Arborist - DZOFilm 20-55 T2.8 Lens Test (w/ BM URSA Mini Pro G2) by Ryan Pham (4K)

Plus, it comes with a 1 year warranty.

Hope this is helpful and good luck finding the right cine zoom for your needs!