This may not be a great proxy. There are schools with high boarding percentage where most kids leave on weekends, because they are more local, because it’s the easiest way to avoid the rules, etc. There are schools with lower boarding percentages that manage to get many day students back on weekends.
I definitely agree that as you do visits, this is something to explore. It’s not just whether people stay, but what they do - and what the flexibility is around that. If it matters to you, I’d also ask about costs. Do you like the weekend programming? What do you do if you don’t? What if you just want downtime to relax? This is really individual.
I agree that you have a great list for exploring, and that is really one of the best things about the application process. It is a privilege to get to figure out what you want your 4 years of high school to be and then to make them that.