Design on-call is a tough process that constantly changes conditions. The number of people in teams changes quite often, and you can add more people during the hiring and remove people from the team, and that isn’t easy. We don’t discuss handover time for sites and what is recommended.
If you choose Wednesday as an SRE book, I recommend changing to a weekly shift or Sunday or Monday. We use Sunday for reporting purposes, and we have an incident review on Monday, but I don’t see this as a problem. Some recommend making changes on Wednesdays, not overlapping with development planning or other company regular meetings.
There are time zones that make you think about different times for handover. We had two sites with California (9 am PST) and Central Europe (6 pm UTC+1) was a good time for us. But some combinations can’t work as well. For example Europe and India, the time zone difference are too small. It’s good for cooperation but not suitable for oncall. Sometimes it is good to don’t tie development teams to oncall teams.
Good luck with designing your first on-call!