You most likely have not damaged yourself, but I would not recommend that you perform this practice of applying pressure to your penis during ejaculation (so I believe that it is good that you have stopped). You are correct that the route for the exit of ejaculate doesn't have anything to do with the firmness of your erection. It is, also, normal for the tip of the penis to be less firm than the shaft during erection. So your penis is most probably completely fine.
If you wish, you could always set an appointment with a urologist - these are physicians who specialize in the male genital system (among other things). They would be able to give you a thorough exam and confirm for you that everything is as it should be.
I hope this helps. Please let me know if you have any further questions. Take care.
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