My question : is Bitcoin spendable in the first place ? Is it designed for and only for HODL, to provide a proof of wealth/stake ?
It is spendable. Treat it like your serious purchase fund. We all have things that we buy daily, weekly, monthly... and things we buy maybe once every decade. Some things are paid for once in a lifetime, like your child's school.
The second : I always heard the "be your own bank" among crypto features. Will the next step is to be your own CENTRAL bank ?
Central bank issues currency. We cannot issue our own Bitcoins, so that should answer your question.