Septuplets - it's even more difficult - but even more inspirational - the rhythm group.
You need to divide the main note of the bar into 7 equal sections, which requires a lot of skill in playing and "feeling" this type of pulse.
Exercises with septuplets are, however, very developing and interesting in terms of sound, so it is worth practicing such groups, or at least try to get used to them more.
The principle of "getting used to" is similar to working with Fives (Quintuplets), so we will use the already known exercise schemes, however for sevens.
1. Finding the right pulse - play sixteen note triplets with 32 notes interchangeably and find the "between" pulse, ie Septuplets.
2. Use the single strokes pattern of the hands + accents - starting the group once from the right, once from the left hand and emphasizing it on the first note of group, it will be easier for you to find in time.
3. Move one hand from the snare to the other instrument (e.g. right to floor tom) to hear the sound of septuplets better, i.e. master the sevens.
4. Try to add a denser hihat (eights), or help yourself with your foot stomping.
It's difficult at the beginning because the first hitting the leg falls on the first note of a group of sevens, and the second eight played with the leg exactly in the middle between the fourth and fifth notes of the group (7/2 = 3.5).
When you manage to freely hit the foot of eight, playing with the hands of sevens, you will not only get a quite new level of coordination independence, but also better settle in the pulse of septuplets and the leg will help maintain this pulse.
Good luck, it won't be easy, but I promise you interesting!