Ah, but the Congress CAN set rules for itself. And the only branch of government that says you cannot reduce their pay is the judiciary.
Yes, they can, but what possible incentive could Congress have to REDUCE its pay? We’ve seen how corrupt the Federal government has become since the 16th/17th amendments, and that’s because they’ve got both the power (17th) and money (16th) to do pretty much anything they want, ignoring what the states want. The last power the states have over FedGov is the ability to introduce Constitutional amendments. And that one is not being used.