The trick is you’re doing it backward. For a lot of tools, the cheap tools aren’t much worse than the expensive ones unless you are at the level where they are in constant/heavy use, hours of driving from the store, and you desperately need the extra durability, but the reliability of the batteries is where it’s actually worth the cash. Good batteries can hold a charge while not being used and last longer while in use. Cheap batteries can be e-waste after a week of use, or even before you open the box if they’re really bad.

As for ethical? The question is absurd. They’re your tools. It’s unethical for companies to even sell tools with proprietary connectors because it effectively acts as them telling you what you are allowed to do with your tools. There is no ethical problem in bypassing corporate B.S.