I’m with ok4450 on completely ruling out a plugged catalytic converter before doing much else. Usually, pulling an upstream oxygen sensor will allow enough exhaust pressure to escape to make a significant difference, but may not have in this instance. A vacuum test is definitive in diagnosing this, but another tool-free test, although not entirely definitive, is to rev the engine while in park. If the engine falls flat on its face at a rather low engine speed (probably under 2k RPM from the sound of things) and sounds somewhat like a vacuum cleaner, there’s a very good chance the catalytic converter is clogged.