I’m not sure I get your request. A Keylogger 'logs your keys", e.g. your keyboard input. It doesn’t matter what a application does with the input (you called it ‘obfuscation’). The weak point is not the browser or the data transmitted to your browser but the physical input that is logged. I’m not sure how you want to protect against that.
So, if the “additional input” you mention has “always to be the same”, it’s trivial to log that input too. You might think that it would require a “bitwarden specialized” attack rather then a generic keylogger but modern keyloggers log more then just key input: mouse movement and screenshots can be captured just as easy and are often used.
What you request is “security when the input device can not be trusted”. But as of today, no encryption method can protect against such things. Only authentication, e.g. using FIDO2 with keys protected on a secure enclave, can do what you wish. But authentication is not enforced (as you correctly mentioned) if you have access to the encrypted data.
What KeePass (with some addons - details of the implementation varry and from my research in the past years only a few actually offer a higher level of protection) or KeePassXC do is to re-encrypt your data with a new, random key every time using a composite key made of your master password and a random key. The random key is encrypted using the “next” OTP of whatever auth system you use (might be TOTP, might be something else - doesn’t really matter). Because of this you always need to “authenticate” in order to access your data. However: this actually just looks more secure. Because most people forget something very essential: we’re doing all that because we assume the system you’re working on is not to be trusted. If that is true however, then the attacker has not only access to your most recent copy (encrypted with a random key that he not yet had access to) but also the copy before, that you just recently accessed using your authentication (which the attacker now also has). Thus, while he may not access the latest version, he may access the version before.
If you can come up with an actual encryption that can not be attacked by logging your activity on a untrusted system you probably will win the CS award of the year.
So, don’t get me wrong: we all would like to have such security. But it doesn’t exist (yet). And what you suggested (and other solutions implement so far) doesn’t provide any meaningful solution to the initial problem that you have to trust your computer.