Have you tried going for "blackmail her" during the Midnight school scene? I haven't yet but I think that's the route to unlock her other scenes
READ THE LAST 3 SENTENCES FIRST!!!
Good news!!! I've successfully found a way to recruit Tsuyu in cave exploration!!! Google Drive with the files inside dowload both the files. Now, after downloading, search Renpy Repack then submit the downloaded "2-1-LT1" below that you will need to submit the "security_keys.txt" which you can find in your save folder of the game then click "repack and download". After that, you will have "2-1-LT1-1" in which after you obtained you will need to delete the original 2-1-LT1 and rename the "2-1-LT1-1" to "2-1-LT1". Lastly, you will need to insert the new 2-1-LT1 and the "persistent" files inside your save folder of the game then launch the game and click "Load" and then you can find the new save file in page 2 of the "Load" page.
WARNING!!! YOUR PREVIOUS SAVES MIGHT GET CORRUPTED BECAUSE YOU CHANGED THE PERSISTENT FILE, I RECOMMEND FOR YOU TO CREATE A BACKUP OR CONTINUE THE NEW SAVE FILE!!!!!!
Ps. This is the "Join her" route.
Ps. I saved this from 0.27 so you might need to play more onwards