I think it's a mistake to frame the evaluation as a comparison to your opponent, but it's an easy trap to fall into (I still do it all the time). In most games the decisions available to each player are so wildly different due to randomness or a few early choices that the comparison is meaningless.
Openings are the place where the comparison with the opponent are most fair. Here you are mostly looking at opening + first couple reshuffles and you just have to be super honest about how lucky/unlucky the shuffles were for each player to decide which opening was better and by how much (if they were different). This isn't really useful, I know, but hopefully anyone reading this has some idea of how to see if a deck is headed in a good direction. It may help to look at a game some time after the fact to boost impartiality.
During the other parts of the game, I'd just try to compare my choices to some ideal perfect player, whatever that is. Which to me just means, ruthlessly consider every decision point as if you might have made a mistake there, and think about what the better things might have been. I'd also say that you don't need to come up with a final answer either, you don't need to actually figure out what the best play is, just the act of considering more options and trying to evaluate them critically will help you improve.
And everyone, all the time, makes plays which are "objectively" bad, in the sense that doing something different would have gotten you better/more Cards, or avoided a terrible reshuffle, or ended the game with a win immediately. These are often related to playing chains of Actions in the correct sequence, or just being very careful with your endgame arithmetic. All it takes to find these is a bit of patience and consideration (either during or after a game). They are often fairly clear-cut, I think most experienced players can identify them with practice, and can often make a big impact on how good your turns are.
And just try to be really honest with yourself about how shuffle luck impacted the outcome, I dunno how to do this really, "just try" is such lame advice but that's what I've got. I guess one trick would be to intentionally only look for bad luck on your opponent's side for a few games, just focus on how they're getting screwed over by shuffles. Sometimes it's a lot easier to see this stuff while watching other people playing.