
Besides adding in new songs and music videos for those songs (which happens about once a month) and adding in new mechanics of event types (which from the looks of it seems to happen twice or thrice a year) a whole lot of the core gameplay doesn't really change thankfully. I imagine with programming, their work goes more towards making sure everything continues to run smoothly, and bug fixes (a couple of which we got notices for this morning, apparently an item wasn't dropping from a song when it was supposed to) and ensuring compensation for people affected. It's a running joke that with any kind of gacha game, the devs will give players free gacha currency when they fix their bugs, so you'll often see people ask for currency over the silliest things.
At the heart of any idol game like this is the artists though, their work is constantly amazing and can't be thanked enough, I've seen the quality of art for multiple games go up over time and these new costumes and art and everything come out constantly on a consistent schedule, and it always looks amazing. Here's one of my favourite Setsuna cards as an example:
I believe work laws in Japan aren't the greatest and people can often end up being pushed to overwork, but these people are doing an amazing job on all these games, not just this one, and they deserve all the recognition and relaxation. I'm sure every person does get breaks, but you don't often hear about things from the staff, especially for things like that.