Pathfinder: Wrath of the Righteous

TL;DR

The difficulty mode ranges from story to unfair, and there are custom options, sliders, and drop down menus to adjust the difficulty parameters individually
The cleric in base sells scrolls of atonement. Alignment is weird in that when at the extreme edges, good/evil will push you along the boundary to the top center or bottom center, so someone Lawful Good taking many good options would find themselves shifted to neutral good in a matter of time
Lore Religion is important for the resting mechanic, Perception is highly important for everything. In general you want a skill balanced party.
Make back-up saves
Most everything in the game has some type of damage resistance, spell resistance, and energy resistance/immunities. Look for abilities and consumables that can bypass DR, have casters take spell penetration feats, and Ascendant Elements will ignore it entirely.
For weapon enchantments, +3 will count as cold iron to bypass DR, +4 as adamantine, and +5 will bypass aligned DR (good/evil). This doesn't apply to regeneration.
When facing tough enemies read their stat sheet to see if they have immunities to spell types. Something immune to death spells will ignore Finger of Death for example.
Metamagic and consumables can make challenging fights easy. Use all the tools and tricks at your disposal when hitting a wall.

There's some reactivity for your race and deity worshiped
Some Mythic paths favor certain builds, but all have feats and abilities to work with any class
Any single class build should be sufficient for normal or below.
Pets are incredibly powerful, and you can also get them from creative use of mythic abilities or class dips.
Prestige classes sound amazing but aren't generally better then base classes
You get several free respecs from Hilor at base.

The game rushes you along but is very generous with any consequence timers. The exception is Act 1, where some things will play out differently if you take a few days to get to them
Companion quests aren't timed but a good idea to do when you get them for the rewards
Each chapter will telegraph its end and ask if you are ready, and you'll get one more chance to decline
Mythic paths need to be unlocked to be available. 5(Angel, Demon, Trickster, Aeon, Azata) are available in act 1, Lich is available in Act 2, and the last 4 are late game decisions requiring a bit more effort in Act 2 and on.
You can't complete Heart of Mystery or Nenio's personal quest until Act 5.
Crafting in the game is useful, especially with making high CL scrolls of break enchantment, remove curse, etc.
When Storyteller disappears in Act 3, he can be found by following the Dragon quest.

Outflank is great on any melee companions
Dazzling Display is required for Shatter Defenses, a great combat feat if you have the ability to make enemies shaken (Dirge Bard)
Taking class dips should be planned out for what you gain vs lose. Blind experimenting is not recommended.
Long term, casters need: spell penetration feats, ascendant elements if doing elemental damage, anything that increases CL of their primary spells, and feats that boost ray abilities
Depending on your build, some spells become more or less important. Someone who does not bother to take relevant feats has no use for rays, spells from your prohibited school as a wizard are likely not worth two slots. An arcane bloodline/sage sorcerer who focuses on Illusion can make Phantasmal killer/Weird work consistently on harder difficulties. Play to the characters strengths.
A list of suggested spells

Melee/Ranged need weapon focus/specialization and the equivalent mythic ranks to boost attack

Check which mythic abilities scale off of rank and which don't. Abilities that increase at rank/2 are better at later levels
Full Divine casters can combine spellbooks with Angel, and Lich can combine with Arcane. This results in getting 9th level spells much earlier.
The mythic spellbooks do not benefit from abundant casting
Despite the first mythic power you take, you aren't locked into a mythic path until the end of Act 2
You can generally get mythic ranks by doing the mythic quests as soon as you can
Taking second bloodlines and impossible domains can be extremely powerful when done right

You'll get most of your companions by the end of Act 2.
You do not get a bard companion
Hiring 1-2 generic NPCs from the Pathfinder in Act 1 is a good idea, they start off only costing 2k gold at level 3.
Depending on mythic path, you may lose 1 companion at the start of Act 5.
You may lose companions depending on how seriously you take your alignment
The Lich can recruit 5 undead companions. 3 in Act 3, 1 in Act 4, and 1 in Act 5. They don't really talk or banter much.
You can romance some non-companions if you choose

The timer is very generous
A strong starting army is 350 archers, 350 footmen, 70-100 Hellknights, and 70-100 Clerics
Offense is better then defense, and ranged units are extremely good in sufficient numbers
Generals need cure wounds, fire bolt, and +1 to army size. Choose those options as soon as they appear in level up
Setsuna Shy is the best general for those abilities right away
Late game upgrades to champions, marksmen, and hedge knights seem to be the highest damage dealers
Small unique unit stacks are useful for scouting out the map, but aren't effective in combat
There are guides that tell you what each relic decree will produce, otherwise its just more gear
Buildings add logistics points to the rank, and can level you up faster the more you build.
Some events can be saved for a quick Crusade morale boost if low

The secret ending requires over 40 separate choices made throughout the entire game and you need to fight the final boss during a specific week of the in-game calendar. It's a completionist goal not something you're likely to accidentally do.
Crafting spell scrolls for ones that require reagents is one way around the reagent cost. Dinosaur bones for example are pretty limited in game.
Check every vendor's full inventory. Some have extremely good gear you won't find anywhere else that might last from Act 1 to the end of the game (e.g. Ring of Icey Protection in the Tavern).
There are 6 magical essences in Kenebras, 3 in quest areas, and 3 in Gray Garrison. Getting 5 and talking to Storyteller as soon as you can after Act 1 finishes can get you an extremely powerful belt slot item that you are then shortly prompted to give away.