Is there a way to shrink your testicles permanently without killing them? and keep being able to get erections.

Ethyl alcohol injections. You CAN kill them with that but seems to be difficult.

will that be permanent? lower my testosterone? which other side effects my experience other than shrinking my testicles?

I found spironolactone definitely shrank my testicles but did not reduce my Testosterone. It blocks rather than reduces. It did stop the testicular pain I have been having.

Given the anatomy of the testicle I seriously doubt you could shrink them without causing long term or permanent damage. You could elect for castration and choose child size prosthetic balls if you wish to maintain the feel and appearance, then control your libido with testosterone.