This is a pretty tough one to answer. There are so many graphing options. I had never even heard of “Plotly graph card” before but it does look neat. There’s also ApexCharts card, mini graph card, history explorer card, etc. Or getting your data into grafana and then doing dashboarding there.
If you’re just getting started I would say just stick with the OOTB cards personally. Like it sounds like you’re literally just getting everything hooked up. Do that first, get everything connected, automations set up, etc. Then dig into your dashboards. If you find you can’t make what you need with the OOTB cards then go looking for a custom one that works for you.
This explains that better then I can. There’s a section in here on long term statistics.
This depends a bit on how you installed HA. If you are using HAOS or on a Supervised install then you should be able to go to backups and take one. Note that by default you only take backups manually and they are only stored in the /backups folder of the same machine. If you want a backup schedule and to have them uploaded to a cloud storage provider then there’s a couple addons that can provide that. There’s a google drive backup add-on for google drive and a nextcloud backup add-on for nextcloud. There’s probably others, can search around for your cloud storage provider of choice.
If you use core or container then as of 2022.4 you have a backup option as well. More info on it here.
Another tough question with no real right answer. What I would suggest is look up supported devices for each of those. Make sure all the devices you intend to use are listed and all the functionality you expect to be there for those devices is available. If you already have a coordinator, make sure that’s listed as supported as well in the one you choose.
Beyond that its really just personal preference. I use Z2M personally but I can’t really say its better or worse then the others because I have very little experience with the others. I personally like that I can see everything about it in MQTT but others hate that MQTT is involved at all so it’s really preference.
If you are still in the consideration phase for Home Assistant and deciding between it and another platform then that might factor in. ZHA is an integration in Home Assistant so you can’t migrate what you’ve set up to another platform. Z2M and Deconz are separate pieces of software that talk to other Home Automation platforms besides HA so that would likely be an easier transition.