Advanced beginner: Kind of lost on what to do now

I just finished the Whisper in the Walls quest chain after re-starting playing Warframe a month or so ago. Last time I played was around the time Plains of Edolon was introduced so its been a while.

Anyways I tried to do some quests in Zariman and it was obvious my Operator was too weak. So I'm grinding the Plains for a bettter Amp. But I have no idea if I'm supposed to even be doing this. Right now I'm mostly playing Saryn and Mesa. Saryn is far and away my better equipped Warframe but if I try and take on more challenging content she gets wiped out pretty quick. I want to get Nourish on the Mesa but, again, I don't know if that is what I should be doing to try and take my frames to the next step to take on harder content.

So I guess I'm kind of lost on how I should be spending my time because it seems like there are infinite things I could be doing. I could finish out my Codex which seems like a pretty straightforward thing to do, I could be trying to grind resources/rep in Plains to buff up my Operator, I could be grinding rep in Demios to get the ship part to eventually get Nourish on Mesa (would also need to get the frame with Nourish which seems complicated as well.) But I don't know what is recommended for someone like me who is MR10 but seems slightly stuck at being able to take on harder content with my current frames.

Any help/tips/advice is appreciated. I have no shortage of things to do I just want to make sure I'm doing the right things.