Hello. Ah yes I want to also try out Thundercat in my next build! So for your questions
1) Druid is really versatile and I don't think you can go wrong. I chose SC Druid w/ Ancient. I would recommend playing a class SC the first time around to really know what the class is capeable of, and then decide which multiclass is suits your goals. The other thing you can do is pick up Tekehu, a companion who is ca be a SC Druid. He, however, is a unique subclass that cannot use fire spells, so you will be missing out of a few of my favorite Druid Buffs, such as Form of the Delegman.
2) The Lightening storm spells are nowhere near as powerful as they were in POE 1, but they are a worth casting in a battle. I usually pick up the healing spells along the way, Insect Swarm, Embrace the Earth's Talon, Weather the Storm (+5 element AR is huge), Entropy (I like it for boss fights), and of course at Powerlevel 9, Greater Maelstrom, which is incredible. For a lot of the spells I mentioned, you will need to debuff Fortitide quite a bit, which tends to be high for most mobs in Deadfire as I recall. However, you will have other tools at your disposal to debuff Fortitude. Actually, Spreading Plague at PL 3 is a good one to begin with before casting the other ones.
3) As for sygnergies, there is a post on the forums now about building a chanter/druid to interrupt enemies. I would check that one out (it is on page 1) sorry I am lazy to go b