RE: Druids and Shamans
For me/my setting and system, they are easily distinguishable for a few reasons. Some is setting fluff that isn't really relevant for your purposes.
But the definitional fluff and mechanical distinctions would probably be helpful/translatable for you. If nothing else, maybe they will give you some jumping off point or elements to consider for your own classes.
As I mention above, both Druids and Shamans exist in a category of those magic-workers who rely on an energies draw from or directly granted by an external power source. Casters who "invoke" their power, a large degree through "Channeling" that external power. For my game, after a great many years of cataloging them as "Priests," I changed the group name to "Mystics" (again, as opposed to "Wizards") and it has made classes fit more obviously and feel more different from the Wizardly (predominantly Arcane) magic-workers.
Druids are Nature Magic users. They have always, for me, occured in a space between and combining the Cleric (they get some armor, some weapons, channel as a primary feature) and the Mage (advanced knowledge and spell use).
They gain abilities from channeling the forces and power of Nature, itself, rather than any personification of elements of nature (deities) or any other spirits (fae, elementals, etc...) attached to or born from it.
Most of the class features we (D&D/PF/fantasy RPGers) think of as "Druidic" powers: shapeshifting, avoiding enchantments of sylvan creatures, enhanced protection from elemental (fire, cold, electricity) energies, including some traditional spells like Pass Without Trace; are actually "Channel Nature" abilities for my Druid class.
They also, clearly, have a spells/a casting slots progression chosen from a "Nature Magic" spell list, as typical/one might expect. Your Obscuring Mist and Faerie Fire, Call Lightning, Summon Nature (Allies), Tree Stride...all the greatest hits. Their spellcraft is "cast" as any Mage would do: incanting special words/phrases and gestures of specific formulae in a secret/sacred/forgotten/unknown ("Druidic") tongue.
Fundamentally, the Mystic spell lists are shorter and more narrowly defined than the Wizard spell lists. Mystics get armor and weapons and other features that Wizard classes just don't. Their (wizards) whole shtick is the magic-use. Mystics use/have a good amount of magic...but they aren't helpless without it.
ANYwho, so, Shamans are also often thought of as "Nature Magic" users. However, what is more accurate is that they use "Animism" which sits -again for my/my setting- in the Vin diagram between Nature and Divine magic. The nexus of which allows greater/simpler access to "The Spirit World" than either "way out from the outer planes Divine Magic" or "actually accessed right here from the Material or neighboring energetic and elemental infused planes Nature Magic."
SO a Shamans primary feature is the Channeling of "Spirit(s)" (rather than the Divinity of Clerics or Nature of Drudis). This permits them features that may duplicate either Clerical or Druidic channelling features (Effect Undead, Shapeshift, etc...) and some things that are their own, such as Summon[ing] Spirit [Entities] for various purposes.
Their spell casting functions through the contacting and contracting with -specifically, "entreating"- various spirit entities. This gives them a spell list that can incorporate spells that appear on Divine Magic lists (entreating with positive energy and higher plane beings for blessings, healings defense, etc...), Nature Magic lists (entreating with elementals, fae, spirit animals, etc... for Communing with Nature, controlling animals, conjuring the weather, etc...), and Arcane Magic lists (entreating with just about anything, including darker forces/lower plane types for charms [fae or devils], levitation/flight [air elementals], necromancies [shadow spirits, demons], etc...).
Their spell list is as short as any other Mystic -as far as spells per spell level is concerned. But they have a broader diversity than others....perhaps even moreso than bards (which are considered a Mystic class in my setting/system).
So, while a great many people conflate the shaman and druid or presume one to be a subset of the other, both animists, and so on...which may be entirely defensible and even [potentially] historically accurate... I differentiate them that way.
Druids are Mystics of Nature. Shamans are Mystics of the Spirit World.
Do/can their magic overlap or do similar, if not the same, things? Yes.
Are they the same/identical? No.
Do they enact their magic in the same processes/ways? Yes -as both channel, and No -channeling different sources, casting by incantation vs. entreating external entities, having different spell lists, etc...
SO, yeah...there's a longwinded, largely unasked for, post 'bout that. I need some lunch! lol.
Good luck and look forward to where you go with it.