We don't know anything. If you want to know how we are sure of it, we aren't.

But the information that light always move on the same speed, on any direction and for every observer has been tested again and again since the end of the 19th century. Nobody ever saw it fail.

There is of course a huge number of variables nobody tested yet. And we can't know if those are important or not. But we are very logically biased into assuming they aren't until we have a real need for them to explain something.

(Anyway, the thing you are trying to invent is a kind of modified Newtonian mechanics - even though it's not quite Newtonian, but people use that same name. There are other people trying to get a workable theory with similar parameters. Up to now, nobody got any.)