If the focus is on "casual content", then my advice might be…
About the year 2022
That's current.
Endwalker released at the end of 2021. There's no special reason to assume that guides written after that are completely worthless.
How to work with your co-healer
Assume your co-healer doesn't exist. React to every mechanic as if it's all on you to heal through it. Adjust based on what you see them actually doing.
For example: If you have a Medica II or Succor spammer, maybe you can take it easy on the heals. If you have a "I only raise when Swiftcast is up and GCD heals are for losers", maybe you need to be a bit more active. In the absence of chatting and making a coordinated plan before the pull, you will need to adjust on the fly.
How to play cards
Yes.
It's casual content. You can't assume anyone's try to execute the optimal rotation for their job, and even they are, you can't assume that they aren't horribly drifting buffs and whatnot. And this is to say nothing of the differences that lack-of-level-sync and gearing might cause.
Either you have a third-party tool to tell you what your party mates are actually up to, or you don't. And whatever a guide tells you, you'll still have to adapt it to the party you actually have.