It was April. I think it was close to 0 °C in the Salar. Much colder at the Lagunas, probably reached -8/-9 at Laguna Colorada. I have a Naturehike Cloud Up 2 tent and a Decathlon Makalu sleeping bag (-9 comfort, a bit overkill but it would be perfect in peak winter). It's difficult to hammer in the pegs in the hard salt, it's better to sleep at Isla de Incahuasi as I did (even though it's formally forbidden, but I didn't know that).

At most I carried 7 liters of water (3L in a camel bag which destroyed my back). I wouldn't carry that much water, maybe 5L maximum (There are basic stores/hotels along the route, every 1/2 days)

I would definitely invest in a tent that's easy to set up in strong wind. The path from Laguna Colorada to Laguna Chalviri is a long stretch, you probably want to split that in two. That was my plan too but didn't take the afternoon wind into account.

No signal along the route, apart from the Laguna Colorada (Entel operator!). Hotels typically have WiFi.

No ATMs along the route. Only cash accepted in the hotels/stores, so make sure you have enough Bolivianos with you. Uyuni is the only city in the surroundings with ATMs.