My question : is Bitcoin spendable in the first place ? Is it designed for and only for HODL, to provide a proof of wealth/stake ?
Why not! It's very much spendable provided there is another party ready to take it! You won't need a mediator like bank here and you can directly send the bitcoin to anyone's account id. As sim0le as that! I am not sure why you even have such questions!
The second : I always heard the "be your own bank" among crypto features. Will the next step is to be your own CENTRAL bank ?
This statement simply says, you do not require a middleman to send money to anyone! If you own the private key, you own the money and no one else does! I am not sure what do you mean by "Central Bank" here! On a funnier side, if you are a miner, you can call yourself a central bank!