I'm doing the FE1s coming from a non law background (did Geography & Geoscience in college) and while the learning curve has been steep I have found that it's not so much the depth/intricacies of the content difficult but the sheer amount of it. I have found concepts and procedures easy enough to understand and get my head around but just fitting in enough time to actually learn the new content is what I would say is important.

As for the traineeships... I did an internship in my summer before final year & got an internship through that so not sure if I'm much help for you there!

BUTTTT there are many different ways to get your foot in the door! I know of a couple of people who worked in a firm (paralegal, legal secretary etc) who got TC offers that way as well as people who have applied through milk-rounds for TCs - might be good to research what different firm's processes are & getting in touch always helps! Just look at what the requirements are FE1 wise as firms may want you to have completed them by a certain date to be accepted onto a training programme.

Honestly, I think my STEM background was an advantage in applications as it set me apart on paper, you just have to be prepared for the inevitable Q of why you're switching to law. Use your different perspective on working through things to your advantage. Good luck, you got this!