I just restringed two of my blinds a few weeks ago. This is what I ordered.
Mandala Crafts Blinds String, Lift Cord Replacement from Braided Nylon for RVs, Windows, Shades, and Rollers (1.5mm, Black)
https://www.amazon.com/dp/B07LC323R7..._cpaBFbWJWRM7E
1.5mm is slightly thinner than what the factory used. This cord feels like it has a wax embedded into it too. It tends to stretch during the first few days after installing so you will probably have to tighten them once or twice. The key is to only set enough tension to hold the blinds up. There shouldn’t be much resistance when raising or lowering them.