Glad to hear that you're enjoying what we have and that you're progressing in your practice.
The only way to gain strength is repeat, repeat, repeat! Muscle strength builds over time and they naturally adapt to the demand that you put on them. So repeat the classes that push your arm strength enough so they are forced to improve.
The other aspect is flexibility. It can feel like your muscles are not strong enough when you are actually pushing into the limits of your range of motion. So, in essence, strength and flexibility go hand in hand.
Do you have a few classes that you already have in mind?