I think that there are some important elements to consider in order to make an informed decision. First, you should think about when would be the best time to get a tutor. Since you are currently in year 10, you should consider whether having a tutor is a priority, or whether you should get a tutor at a later stage, such as in year 11 or year 12, with year 12 being the year where everything matters.
Second, your current performance in Stage 5 Latin serves as an indication of your performance potential in Stage 6 Latin. Since your performance has been of a relatively high standard, it can be seen as a positive indication, based on which you may not need to get a tutor. You mentioned that you have been recently struggling with grammar. You should first see if you can address the memorisation problem through your own efforts as well as by discussing the problem with your teacher and implementing their suggestions/feedback. If that does not work, then having a tutor would be a good idea, since they can provide guidance and support, allowing you to address areas of improvement. On the other hand, studying at the last minute is an issue that you can definitely address on your own by developing and committing to an effective study plan/strategy.
Third, you should consider whether the other person who has a Latin tutor is benefiting performance-wise. This is because getting a tutor may not always enable you to improve your performance, especially if you do not clearly know your weaknesses, which your tutor must focus on in supporting you. Sometimes, you may find that you can address performance issues on your own. Whether this applies in your case can therefore help you determine if you should get a tutor.
I hope this helps!