Hey everyone,
I’m a long time Japanese learner. I’ve learned Japanese on-and-off for the past ~10 years, finally passed N2 last December and am currently working towards N1 this December. I’m a big fan of Wanikani - been using it since 2013 (I keep resetting my progress every few years as I restart my Japanese learning journey but I’ll get to level 60 this time!). I’m also a big fan of the other tools we have in the Japanese learning community - been using Bunpro extensively and recently Kikikata for listening.
However one thing I felt I’m missing and was really hard to do without a Japanese instructor was practicing sentence construction. That’s why I’m starting to work on a new app called Sakugo, designed to help me practice and get feedback on sentence construction. It’s a hobby project - I’ve been working on it during weekends for the last couple of weeks - but I don’t have that much time so it still has a lot of issues I need to fix. I’ve been building it mostly for myself, but it’s at the point now where I’m actually finding it pretty useful so I thought maybe I’ll share it and get some feedback - do you see yourself using this tool? What works and doesn’t work, and what features would you like to see added? I’ll be really happy to get your thoughts!!
Note that the app is using an ML model at its backend, so I can’t offer it for free in the long term (as I need to pay some amount per practice session), but I’m opening it now for ~50 users to beta test for a limited time (sign ups will close after the first 50 beta users). I would be very happy if you can try it out and tell me what you think!
You can sign up at: http://sakugo.app/
And please be forgiving to the many bugs you find! 
P.S. Please note that at least for now, you will need to install a Japanese input system on your device to construct your sentences. There are instructions for how to do that here.