Dzungars could have included OTL Mongolia, Kazakhstan, Kygyzystan, Xinjiang, Tibet, Qinghai as well as some part of Inner Mongolia, Northwestern Gansu.
You need
1. Keep intact Khalkha-Oirat alliance intact and prevent Khalkha Mongols voluntarily submit to Qing Dynasty. Over time annex Khalkha Mongols.
2. Make Kalmyk Khanate established on eastern shore of Caspian sea, approximately OTL Western Kazakhstan. Them submit to Dzungars or will be conquered.
3. Keep Russian and Dzungars allied. I think Russia is no interest in fighting strong Dzungars, as most of attention will be expanding in Europe and getting warm port on Baltics and Black Sea.
4. Make Dzungars modernize with help of Europeans like
Johan Gustaf Renat
I agree there is possibility that Russians would swallow most of Dzungaria post Napoleon War.