Set the stops right, make sure the cable and housing move freely, and make sure the rear loop of housing isn’t too short and causing friction.

Its $15 worth of housing and cable plus your time.

Also check the chain to make sure there aren’t stiff links. Make sure the crank and rear wheel are not wobbly.

And that is all you can do really.

Don’t get nice tires, just replace worn out with similar cheap ones.