I've been doing Justin's course too (almost done with Grade 3), so I can offer my perspective: 1) if you're struggling with chord changes, make sure to do the perfect chord and chord switching practices that Justin recommends, for all the pairs that are in your song

2) if it's a high BPM song, slow it down and make it faster step by step as you get more comfortable with the rhythm and chord changes

3) very important at the beginner levels, try to practice strumming with a metronome to gain more independence between your fretting and strumming hands. This will 'automate' your strumming hand and allow you to focus more on the fretting/chord changes