The key with the assisted Cossack squat is to spend as much time as possible being active in the position to make it more familiar to your body, then ditch the weight and try to recreate it under your own core strength.
Depending on the severity of your mobility restrictions you may need to start with a heavier weight and gradually decrease the load over a few sessions... or you could very well get lucky and have gained that movement permanently.
Make this your go-to movement. Sometimes individual joint mobility drills have their place, but when you're training to move your body as one unit then applying the same methodology to your mobility work just makes more sense.
Plus you're actually going to build muscle with this rather than sitting on a foam roller smashing your calves for half an hour hoping for some degree of dorsiflexion to magically appear.