Your 5th finger ISN'T literally weak
In our body weakness means either muscular deconditioning or exhaustion and there are no muscles in the section of the 3 phalanges that you call 5th finger
I think this is important to understand because it's the only way you can understand what the problem with your 5th finger is since it can't possible be weakness and there's no exercise that will make it strong (like it is for bobybuilding and weight-lifting)
Playing is a matter of weight trasmittion. That's why your finger should never curled or away from the keyboard. Non playing fingers should just rest on the keys and not be kept up or outside the keyboard. That's because the weight you transmit from your body to the upper arm and forearm and finally the hand/fingers can be individually directed to each finger or more fingers
You should practice just keeping your hand on the keyboard in play position and then just raise your whole arm and hand aligned for less than a centimeter letting the hand fall on the keys BUT making sure only the finger you're transmitting the weight to will depress the key, the other fingers just being weightless and resting on the key surface
If you do this for days and in a proper manner you will master the kinestetic sensation of just transmitting the weight through the arms to individual fingers.
Clearly this will solve any mental/physical barrier of the 5th finger because:
Since it's a matter of weight and gravity it doesnt' matter if the 5th finger is smaller and thinner
The problem everyone has with 5th finger is not being able to individually transmit to weight to it while not trasmitting it to the middle finger and fourth finger
When you master the weight trasmitting to allow each individual fingers to play the keys in a fraction of a second the 5th finger just can rest on the key surface and it's ready to play just by having the weight trasmitted to it
For this to work naturally you have to find the correct distance and sitting height at the keyboard so that you hand, wrist and arm are in a straight line and hence perfectly aligned for maximum effortless weight trasmittiong
Proper Sitting at the Piano