Like you, I am also a badminton fan.
Our badminton teacher told us that we can practice swing movements in our spare time, including serving, flying high, hanging and spiking. At the same time, practice striding to the front of the net and retreating. Physical training is also very important. It is a very good way for badminton players to practice skipping rope. In addition, you can also properly practice the strength of the waist, abdomen and upper limbs.
But the most important thing is that I think these training methods are helpful to your technical level and physical training, but the on-the-spot feeling of the game can only be realized through actual combat. For example, I was too busy studying in my freshman year to play ball, so I would easily miss it later, but it will be fine after a while.
And I don't think badminton skills will decline rapidly because of a short interruption. Even if you don't play for three months, you can easily return to your original state as long as you can play twice more after three months.
I hope it helps you ~