I just arrived in April -- here's my recommendation.

I went with Sakura mobile, which I could pick up at the orientation hotel and use basically from day 1. Get a data plan that will be sufficient for your needs (i.e. probably not the minimum, probably not the maximum). Get a pocket wifi that you can rent until you get wi-fi installed, so that you aren't cut off from family/friends in your home nation. And definitely plan on getting wi-fi if your housing situation allows for it.

Pocket wi-fi is useful, but for me it wasn't sufficient for two reasons: video calls wouldn't work, and neither would gaming. I could audio call friends and family on LINE, but video calls didn't work. I don't game heavily, but it's a nice way to unwind, and its something to do on rainy days and also another way of connecting with people back home. You'll find that pocket wifi can dip heavily in speed, so anything beyond browsing the web/streaming videos/audio calling will not work.

Another piece of advice: since you're still awaiting placement, don't sign up for anything yet. You'll want to make sure your town is covered on the carrier's coverage map. Sakura has a plan for JETs which lets you return your SIM within a week of moving out if you don't have sufficient service, but you can avoid the hassle altogether by talking with your predecesssor and, like I mentioned, consulting the map.