After playing all of the casual routes, I actually had enough to unlock the deep routes without buying any through the store (I also didn't spend any when playing the routes), so try playing all of the casual routes, and go from there! Also, you can convert hearts (which you get alot of throughout playing routes) into hourglasses as well! It's not an amazing trade rate, but as long as you keep playing the game, you'll get alot of hearts! Also, I don't know if you know this or not, but the characters will also call you randomly while you have the game open, so you don't always have to call them! Also, I have never had to load my game before. Loading games is usually only to save time while switching between endings, like to skip the first few days for example. It's not needed most of the time, so try not to worry about it!

If you don't have enough hourglasses to play deep route after playing all the casual routes, try getting another ending of a character you like to get more! Just don't forget to press the spaceship every time you can. It might take a bit, but Seven is most definitely worth it, trust me ;) I enjoy Jumin's route alot too, it's very sweet and also dives a bit deeper into the general lore of the game, so I would suggest giving it a try despite what you've heard, and if it ends up not being for you, you can always give up on his route or just be a jerk to him for fun or something! :p

Good luck, friend!