There's a wiki, but due to game's complex nature it isn't constantly updated, so the best way of maximizing character's effectiveness comes with experimentation, which is also the point of the game. But I will try to post an updated variant of what I previously thought but with post-update changes.
Druids main strength comes from simplistic progression, similar to neutral character option, but at a certain point it comes to a cap, at which the only way to upgrade your character become consumable potions that aren't exactly easy to craft or get, as well equipment, which is much easier but becomes harder to do as item levels become higher, especially for boss drops.
But then there's seedlings, strength of which is heavily dependent on magic power, previously it used to count bonus ones from traits like infused and such, but now it only counts base power, so you pretty much have to get magic potions, especially ones that can be made with a alchemist job, which I will talk later. But while seedlings upgrade worse than spirits in terms of reshape which encourages holding on summoning them till later, they do make up for solid characters, like a strong mount, a fast but frail support and a support caster that is a centerpiece of seedling parties.
The most powerful thing in druid's arsenal however is blood magic, which means without killing the king, druid's late game is rather weak, making it effectively an requirement if you want to compete, considering that it gives you access to blood fairies, a very good seedling unit with strong growth, abilities and stats, sacrificing seedlings to non-seedling characters to improve their genes and traits, allowing you to upgrade unique characters, but still not yourself, which comes to another 2 points I have to explain.
First one is why I suggest alchemist. As you see, druid cannot gain stats by growth like regular characters, and eventually you'll reach a point where druid has stats comparable to a mid-game character but with more abilities(and terrible lust damage in spite of having upgrades to it), which means their stats will be surpassed by many late-game units easily without consumables, and alchemist gives the option to make ones that improve their stats, and while items can mitigate that, it won't be enough for higher challenges where you'll need every boost you can get. And since seedlings(and spirits but compared to other characters you are going to need much more magic power)you summon are tied to your magic power, it means more reasons to run exactly that.
Another point is hidden power trait. While not exactly necessary, druid compared to others needs it arguably the most due to fact it gives you the option to get skills like side step, that ensures you can keep up against others, a way to change your type to what elemental power you have chosen at the start, especially crystal for stone, but most importantly, ability to give soul fragment to said seedlings and spirits, which gives you a way to synergize with them without breeding, but due to fact druid is level 5 with no genes to speak of, as well stats clearly not meant for breeding, it means the results will be underwhelming, and fusion/non main character results gets rid of soul fragment, so it is mandatory for them in order to utilize their full strength.
So yes, druid is kind of a character that was decent but not exactly weak previously, and is now somewhat weak unless you use blood magic they get from killing the king, making said step mandatory most of the time, and hidden power, which comes from a certain path in alternative ending is also similarly mandatory if you want them to not be weak most of time. Hard to make it work without good knowledge of the game itself, but until they get buffed, it is pretty much the only way to make them compete. In fact, the male creator is better at melee combat than male druid nowadays for far less investment, with the ritual dagger giving them an extra attack on demand per turn(and better yet, can be spread to other characters via breeding/fusing results), and allocating health/strength genes on them gives a strong attack and health stat, on top of ability to have characters that are hard to get normally like seals much easily by summoning them after extracting genes. And using said seals to get powerful items that make you strong further until you can ensure that you can breed them regularly.