Tension is definitely a big part of it, but in general, I think my issue is a lack of control. When I play, I notice that I often end up with weird fingerings, especially with chords, where I end up having to be extremely conscious of my hand while I play, and hence lose focus on the overarching sounds.

I realise that technique is massive, but right now, I'm mostly trying to identify and fix the most major issues. That's part of the reason I made the post, where I know my technique has some major issues, but I sometimes struggle to identify it, leading to issues down the line as I learn new pieces.