Ah, I see. I think your best bet is to handle this in your theme, because core already has the border set to 0 for all input elements:
So, in your theme, what you can do is override the focused input element’s border-radius in the same place where you are now setting it to a value.
If you want to keep the rounded corners on focus, you could also do something like this:
input {
border-radius: 4px;
&:focus {
outline: none;
box-shadow: 0px 0px 0px 1px var(--tertiary);
}
}
(The outline property cannot have rounded corners, so the above switches to using a solid shadow to replace it.)