Hi, is it possible to use Remix to write an external smart contract to overwrite assembly slots of an already deployed smart contract?

If you have written a contract and somehow can hack the proxy to make it its implementation, then yes

haha, not a hacker but trying to solve a code challenge

What challenge?

its for a job application so..

essentially trying to change a read function from false to true

Ok

You can try to use external info from other contract to chose of return data. If u need to override slots data, initial contract must use delegatecall