...
that can be obtained by setting fixed fees that clients know in advance; and clients who pay must get what they paid for.
No it can't. Nothing controls the fees that pools/miners will accept but each pool/miner themselves.
Yes, bitcoin still has several design flaws that need to be fixed before it can achieve its design goal. Like that one. Or the lack of inflation or demurrage, that got it appropriated by a legion of speculators and snake oil salesmen.
You clearly have something else under centralised control - that isn't bitcoin - that you want - go chase it.
None of what you stated there is bitcoin.