> Do current LLM based agents generate code which is easy to change?

They do. I am no longer writing code, everything I commit is 100% generated using an agent.

And it produces code depending on the code already in my code-base and based on my instructions, which tell it about clean-code, good-practices.

If you don't get maintainable code from an LLM it's for this reason: Garbage in, garbage out.