So I have finished studying N5 grammar but am unsure what to do next … there are many things I haven’t fully grasped and so will need more study.
Do I keep going back over these N5 points until I’m more proficient? Do I just move on to N4 topics? Or a mix of both, introduce new material while reviewing already learnt items?
For grammar I’m using the Tobira Beginning Japanese textbooks/videos and MaruMori 
You’ll truly learn those points once you’ve seen them enough times in the wild. It’s just that having a lesson to recall and possibly review again when you see them while reading etc. creates a much stronger memory for them.
Imo it’s a better use of your time to move on and review as needed.
Don’t do this with N5. N4 and N5 grammar points are so ubiquitous that you will encounter them enough even if you go on to “harder” material.
Better to just look them up if you are truly confused when encountering them in the future in context like @catslikerock recommend