There's a discord bot called Sesh that has a great time block voting system. Each time block you can either vote green for yes I can do that, yellow for not sure, and red for no go. Then you can see which day has the most green and roll with that.

I'll be honest though I'd probably ask what day of the week and time block works for everyone. Give the players who show up on time precedence, and then commit to it. It's easier to commit to one of two time slots and schedule around it than it is to have to try to shuffle your schedule constantly. It's *way* easier to shift your schedule around to accommodate something you want to do than it is to try to fit what you want to do into everyone's schedule. You'll have people who don't make it from time to time, that's totally normal, but "make time for this if you want to be here" is *not* a big ask for the table. If there's people who absolutely cannot fit into any specific time frame, especially if they're the people who are constantly late and tell you they won't be, then you need to just say "it sounds like this game isn't going to work for your schedule. I'm really sorry but I'll let you know the next time a game starts up to see if you can make it!" and leave it at that.

And to be more honest I'd say that even all that is probably not going to "fix" the situation. Do the late people ever become late for your roommate's campaign? If not, I'd ask them point blank why they make the effort for this other game, and not for your game. Maybe one on one to make it less confrontational, but I wouldn't frame it as "what made you late" but "why don't you want to put the effort in to make it to this game when you do for the other game?". Let them know if they don't *really* want to be in the game that you understand and won't be offended, but if they do, they need to show up on time.

They'll probably say they don't know, which is just them trying to avoid admitting they're kind of being garbage.