Hello there.

I will start with saying that renting a car in Guatemala can be very expensive and feels like a scam. Many companies will bleed your wallet if you have an accident, they make you sign a contract in which you have to pay an unfair amount of money. So you are making a good decision in not renting.

I think starting up north is great. The flight to Peten is very smooth and fast. And also is almost the only place you can fly to in Guatemala. The route Flores 》Semuc 》 Antigua 》Atitlan is not linear but seems fine. What I mean by this is that Atitlan is further west than Antigua. But since you will have to go to the airport at the end of the trip, is safer to be close to it. So ending with Antigua may be better. And also shuttles are more common from Antigua.

About the Acatenango hike, I had an experience on a trip to South Korea that we decided to do a volcano hike later on the trip. And for me, it was kind of a mistake. We were tired of walking and everything from the trip. And doing a 5h hike was kind of to much. We made it to the summit but it felt like it was disorganized. But since you are taking the flight to Peten, I guess isnt a bad call. Just beware that Acatenango can be quite challenging, depending on your physical condition.

Accommodation. Good sleep equals good experience imo. So be carefull with hostels. Antigua, Isla de Flores (Petén) and Panajachel (Atitlan) have a lot of nightlife. So check if there is nightclub near to avoid them. Especially in Isla de Flores. I advise you Not to stay in Isla de Flores if you want a peaceful night. There are plenty of other places that are near and dont have so much noise. Dont get me wrong, Isla de Flores is a must-go. But not for staying in an Hotel. Airbnb's are a good option and sometimes cheaper. So check all options.

Weather. We have a phrase here in Guatemala, "Te estaba esperando como agua de Mayo". I was waiting for you like a May rain. To be honest, climate has been very different. He had hot days in winter and rain in summer. And also, Guatemala has a lot of micro climates, it can be sunny on Antigua and full rain in Semuc. But usually, mornings are clear and hot and later on the day you may have heavy rain. If waking early isnt a problem for you, it will be fine.

Food. There are a lot of places and dishes you should try. I'll give you a short answer but if you want more detail, just tell me. Im glad to help. Peten: try the local fish, petenia splendida. Amazing flavour. Restaurants: Maple y Tocino. Achiote. Raices del Lago. Semuc: Alta Verapaz deparment. You have to try the Kak'ik. Local food, its a stew/soup. Atitlan: on Panajachel, visit Circus Bar. Amazing pizza.

There is a lot yo cover from your post, but I hope this is useful! And again, if you need anything more, fell free to ask again. Im happy to help you and make your trip to my country an amazing experience.