Magic can do anything, but science is real. The more magic deviates from science the more power it consumes, therefore the more you know about science the more powerful your magic will become. Knowledge of science has apparently been suppressed by someone to keep human magic limited.

around chapter 8 author decides to throw away the slow build up of his experiments and give him full knowledge. Out of no where we have building perfect scientific experimental setups while broke, getting scientific reagents like H2SO4 and stuff

The start of his degeneration is when the town gets attacked by cultists and his sister gets cursed. The setting is abrupt and we don't know why only 2 kids survived the attack of an eldritch god and better yet his sister was cursed. There's no description of the aftermath either and we get a timeskip. The worst part is he never tells his family (cursed sister included) and decides to solve the curse himself. While the curse does kill early at 30, it's not that close that he had to ruin his childhood for it.

MC is a creep shut in who gets too handsy with his sister. Most of the time he takes advantage of her kindness to him as family. He even disrupts the atmosphere at home, by studying magic all the time. And his dad sensibly tells him to pick a job for himself and plan his future, he delusionally says he will research magic with 0 income or actual progress to show.

When he gets kicked out of home to show reality, he decides to still continue and even steals items from home for it. It gets so bad he collapses from hunger afte wasting his money on research and has to rely on the kindness of the store owner to give him food