Strategy for revealing pregnancy to new department/department chair
I don't think that when and how you reveal your pregnancy (as long as its professional) will have much impact on your career, especially compared to the fact of being pregnant and having another child.

I would advise meeting with the department head as soon as possible and reiterating your long-term commitment to the department. You two have the same long-term goal: your being a successful assistant professor and earning tenure. It is in your department head's interest to be helpful to you.

Before going into the meeting, I would decide what I wanted. You mention wanting to stop your tenure clock for a year. Do you want an unpaid leave if possible? While it is good to be flexible, it is also good to know and communicate what your best outcome would be, in case they can accommodate you.

One outside-the-box solution might be if you could defer the start of your assistant professorship for a year and teach your fall course(s) as an adjunct.

If you have started thinking about grant applications or other things of interest to the department head, it would be fine to also talk about those at the meeting.

Good luck!