I suggest you don’t buy MP before you go, see how you do on the first day, and then decide about adding MP day by day. I would never go without MP, but that is just me and my family’s style. We take 2-3 day trips only, so we need to maximize ride density. We are rope droppers, and we stay on site so we always do EMH/MM also. However, even with all of that, we would not get nearly as many rides without MP. With limited mornings (2-3, depending on the length of trip), we would have a really hard time doing the headliners more than once per day because the waits just jump way up very early in the day (if we limited ourselves to going on rides with only 20 minutes or less of standby wait, as your cousins suggested). We could pull paper FPs like we used to, but that just puts thousands of extra steps on the FP runner, separates our family while the runner goes for the FPs, and wastes time. If you are doing a longer trip, you can ride everything in the mornings no problem. But your afternoons are not going to be 20 minute or less lines for just about anything if you don’t have MP or paper FPs.
Right now is not a great time to look at wait times due to the oddity that is the opening weekend of SWGE, but there is a FP return time tracker on this Board (can’t recall who it is) that has tracked a lot about FP returns and wait times at various times of day for various rides. That might be helpful for you to look at. It will show that not much is going to be under 20 minutes after about 11 am most days.