Investing a larger sum at 20 years old (still in school so I can't invest for at least 5 years)

There is always the option of going with IB till you are 25 and then if you have less than 100k transfer the positions somewhere else. Getting them out of IB is probably easier than getting them out of degiro.

Degiro just feels kind of dodgy to me from what I read on this forum.

There have been a lot of broker-choice threads (example) and people somehow usually end up at IB in the end, I started with Saxo (bad Idea XD) and then later went to IB.