As I understand it, cryptocurrency funnily enough works awfully as a means of transaction, because the amount of processing power required to make transactions is ridiculously high.

I’m non-native as well, although I guess I’d consider myself quite fluent. I definitely pronounce the th in “with” and “that” the same, but “with” with voiceless dental fricative really feels like a common thing among non-natives.

Your examples of “with” and “through” are a bit weird, as the former is voiced and the latter is voiceless. Anyway, you should start using ð as well, like in ðe, ðis and alðough. Maximum confusion for everyone else!

Your old classmates who flunked during the last two years probably didn’t post about it on social media. It’s not reflective of reality, just all curation.

Trust me, you’re doing fine. My bachelor took five years to finish, and plenty of people I’ve talked to have had similar situations.