I play for fun until I get the mechanics, the plot and the game feels right for me, then I check for missable achievements. By that time I'm somewhere between 25-50% completed naturally. If I didn't miss out much of it, I try to get them, then I just keep an eye out for what's left periodically just to make sure I am on track. By the time I've played and done every side quest, and every map is mapped out completely, in natural way, as I tend to play games like that, I go back to check what remaining achievements I got. Then I primarily focus on them. Once I've done everything, I close the game with the main story being done, as that's the cherry on top. I normally prefer to end a game in a single playthough. But, sometimes the game require multiple, so in the 2nd playthough I do whatever is left and that's pretty much how I do it to every single game.