he wasn't living frugally, he just was a shitty boyfriend that only took her on dates to all you can eat buffets and disneyland instead of literally talking to her or showing her his luxury apartment for a decadeSeems like maybe he was living frugally while he invested so he could retire. It doesn't actually say that, I'm just guessing from him mentioning that he studied investment and her acting like he never spent money on her.
The dude makes like twenty billion yen a month and the best dates he could come up with are overnight disneyland dates??
he's a total fucking dipshit without any tact or thought that people love for some reason
he's taken in more magic stones than anyone, which make you smarter, and he thought it was a good idea to propose spontaneously without talking about his living situation with his girlfriend or even telling her his income
do not read the web novel, it's so much worse than the manga version with how absurdly wish fulfillment it feels