Btw, there is one thing about setting up new wallets with the same private keys, which is may be a myth or prejudice. But I have heard this several times.
As an unexperienced newbie I cannot evaluate if it is true or false.
It is the following: private keys change every time when one makes a transaction or receives btc, or even just launches a wallet online.
As has been explained to you, the private keys don't change. The addresses that hold your coins change when you send Bitcoin to someone. Since an address is emptied of all its coins no matter how much you send, importing the private key of the old address will show you a zero balance. Maybe that's the confusion. If there are no coins on the address, it makes no sense importing its private key.
Let's explain it this way.
Your
Gorilla22 address #1 holds 1 BTC.
You have the private key of that address. Let's call it
Gorilla22 address #1 private key.
You buy some gear from me for 0.3 BTC. The transaction is made from
Gorilla22 address #1.
I receive 0.3 BTC, some of your sats are used for network fees, and the rest gets sent to
Gorilla22 change address #1.
A year from now, you decide to use a different wallet. You remember that you had bitcoin in
Gorilla22 address #1 because that's the only funded address you used in your wallet. You also find
Gorilla22 address #1 private key in your personal notes. You import the private key only to see that the wallet is empty and you have no bitcoin in it. But you are forgetting that the address has to be empty because you purchased some items from me and your change went elsewhere. The private keys never changed. They still recover your old address, but it's just an address that is empty now. What you need is the correct change address and it's private key, which you found already.