What’s helped me feel comfortable saying no to project scope creep or decisions that will result in overtime is the fact that if projects run over time then it is a failure on managements side; they are responsible for the resources provided and the end deliverables.
Managers will (often) not tell you to work less, they will always find work to fill the time you’re willing to work so it’s very important you place boundaries.
You can place boundaries in two ways, explicit and implicit. 1) An example of an explicit boundaries would be informing the PO during a standup that their feature request will result in you working weekends which you won’t do. 2) An implicate boundary would be not responding to emails, slack messages or PR outside office hours.