Your understanding is correct. Thanks for an answer. It seems that there does not exist such software, unfortunately.
What is your purpose if I may know? As far as I can tell people already use BTC just fine for transactional purposes. If you want to be the bank in the system, I guess you can refer to centralized altcoin where the developers mint a ton of the tokens. You can also pre-mine I guess, but it would be difficult to make it mainstream if control what you want.
The purpose is to experiment with Ux.
Is there an example of an open source "centralized altcoin" tool(kit)/package? It shouldn't feature imitation of decentralization, immutability, resemble Bitcoin etc.
From what it sounds like, you are looking for free fintech software/code...like a banking app, except with its own native currency rather than money. No, there would not be any free, open source or secure fintech code. Usually this code is proprietary and private, and any open source code is just for example sake, or a starting point. It's not generally just ready to go out of the box.
You would be better of trying to build what you are trying to do with web3 and a token, it would probably be easier and less costly, plus there would be a lot more open source code for you to use via smart contracts and from existing open source projects.