I just read your comment and the 1st thing I have to ask is: Have you read the introduction of the game?

Many of your questions have an answer in there and moreover, it is referred to that the icon below the game clock (the one with the book) is a grouping of the game's explanations ( Still under development because I need to add more information).
- Strengh, why increase it? No need to do it to the max but it can help people who are struggling at the beginning of the game.
- The cost of making potions is too expensive? Several skills can fix this, such as the mechanic that decreases the number of droplets. Another increases the amount that monsters give at the end of the fight, ...
- Problem in the cave with fight monsters?
a. You can run away from a fight after it has started.
b. You can buy magic items from the cave vending machine. There is a teleportation item to leave the cave in an emergency and others that can give advantages. You can buy equipment too.
c. There is a pharmacy in town, there may be potions to cure yourself.
d. There are also other ways to heal yourself such as food and spells to unlock.
PS: The random monster encounters are the basic option of RPGmaker...
- Talk / Flirt / Sex / Sex vulgarly : The 1st allows you to earn friendship points and the other 3 give Love points but according to the corruption points that the NPC has. This part is still in development and each NPC must be customized. You can still see the corruption points you need to have for each choice in "my info about her".
- The quests to unlock the content can be divided into 3 parts. Getting to know each other, seduction and intimate closeness. This was designed keeping the theme of the game "the hero is rejected by everyone until the day he is the only man left and he has a way to corrupt the women he meets"
- Intelligence? there is a school, you have to follow the courses to increase it more quickly. Homework is just to avoid the loss of intelligence.
I could tell you a lot of other things about the game but I'll let you look at the guide in the game and you can also go to the GameFaq. If there's a problem, you can ask me
PS² : The book icon for in-game explanations will be replaced with a question mark in the next version.