Cellular: I'll just add from recent experience that, should others reference this thread, Verizon roaming coverage was just OK on a recent visit. Data was there but never had more than 1-2 bars. I don't remember which services picked up the roaming but it seemed to change a lot. Nevertheless, I spent about $30 for a week to cover four phones with travel coverage, which was pretty good (via Verizon, not a separate SIM). They've become competitive.

Cash: have it and use it at the coffee bars and gelaterias.

WhatsApp: a lot of places use it especially Airbnb hosts, but I refused to download another app and had no problems.

Transit: for what it's worth, I've always thought Rome has some of the worst public transit connectivity. The buses are fine if you can figure out the routes to get you from point to point, but I find the metro useless. I see it as more of a suburban train line; not enough core stations with interconnectivity. YMMV of course. On our recent visit we walked everywhere (staying near Vatican) and used rail and regional bus for subsequent intercity travel. Uber was useless in our experience as were the other Italy based apps for any sort of taxi (we had a group that ranged from 5-10 people, and were willing to split into multiple cars) at pretty much any time of day, and we had a heck of a time flagging down taxis. Hailing taxis and using taxi ranks at the train station were effective as was asking our Airbnb host to simply arrange a car, which was never overpriced. Having said all that, Rome is my favorite city and I'd live there in a heartbeat - so don't take any of this as complaints, just facts of life.... Enjoy.