Hi,
Lemme address each point separately:
1) generally speaking - yes, dev has a list of things to be considered adding to the game, there are definitely new positions, iirc toys as well, but that's all the info I have, no idea whether he's thought of implementing them yet, rn he's focused on the story and other gameplay elements, the night portion will have to wait for it's turn
2) yes and no, more girls will be available to fuck at night, like fmc's friend Kaede, but from what I know, the relationship system will only be available for Yuna exclusively, as she's the main girl in this game, who knows, maybe dev will change his mind as the development goes on...
3) I'm almost certain that there won't be any customization options for MC, again, maybe dev will change his mind, there will be however more customization options for Yuna, such as other accessories and cosplay outfits, or at least there are plans for that
4) once you have a certain version on your phone, make sure NOT TO UNINSTALL it, save data is stored in the same folder (hidden in Android/Data) as the whole game, so uninstalling means deleting save files as well, basically download new update, unzip the .apk file and install it, the prompt should ask you to "update" it instead - do that and you're good to go, also it's good to periodically clear game cache (phone settings, all apps, find REC, storage and cache, clear cache) especially after updating or crashes, as android build is unstable and error prone.
Cheers