i don't have the best answers since i'm still under 30 hours in the game haha. but from what i've learned, here are some general tips.

1. for hiring people, keep a diverse, balanced, set of classes on the sidelines. i categorize them into frontline, dps, support, but you make your own definitions.
2. it's good to let people go instead of paying money to fix them. you determine their value, but it's a decision between looking at their level, negative/positive quirks, stress level, diseases, and class. sometimes i would just push a level 0 to their limit, then just chuck them cuz i needed a healer for that run.
3. for heirlooms, i think the rule of thumb is to start maximizing stagecoach until you have enough for #1, move onto blacksmith and guild at an even pace. sanctarium and other stress relief ones are lower priority due to #2.

at the end of the day if you're playing on darkest difficulty, there isn't a limit to how fast you need to beat the game. you can take as much time as you need to. characters, heirlooms, and gold are always recoverable... at a cost. you don't have to be confident in your choices, you just have to make them and adapt from that point forward.

for stygian difficulty, yeah you should ask someone else for pointers.