Sorry this probably isn't much help but genuinely just search up subjunctive phrases Spanish on google and try and cram as many possible into your work. Good ones that you maybe know of already (that sound natural) include 'No creo/pienso que...' , 'Espero que...' etc (I'm not going to say more because you can easily find them on google)
Also if you're struggling with trying to include impressive phrases what I do is instead of trying to accommodate my writing to my knowledge level, I try and translate my thoughts directly from English to Spanish (what I mean is that if I don't know something I would usually try and find a way around it but try not to do that as sometimes the thing you can't manage to do is a complicated grammar point that would look good in an essay - if you're struggling with grammar ask someone for help)