music wise there are 2 reasons you might consider a sub:
1 - you receiver or amp doesn’t have enough power. The low end takes a lot of energy which can/will effect all the frequencies if it runs out of juice.
2 - your into organ or electronic music with a lot of content below 30-40 hz
So, Music wise In most cases your fine with just the M5’s If you play it at reasonable volumes … I would not worry especially in a small room unless your into cranking it.
Video wise a sub is a big deal unless you have well amp’ed mains which can truly go down to the 20Hz area. It’s the boom, rumble, and whomp of any action movie.
A center is important to anchor dialog / action to the center of the screen for all seats … it’s more important in a small room as there is probably a smaller sweat spot.
Bottom line is that only you can prioritize which to go for first …
