Yo I just did the entrance exam and got my ass handed to me, if you want some first hand experience ask me about anything. I'm almost definitely gonna try again next year with tons of more preparation.

So the reduced entrance exam is difficult, but the most straightforward option imo. They say if you pass the entrance exam, you'll be ready and there's an iirc 90% chance you'll pass your first year. Whereas people coming from the matura have a 50+% chance of failing or dropping out. I can't judge your math, chemistry, and physics skills so you gotta know how far behind you are. Biology is just a matter of taking the time to memorize. Take a moment to browse the ETH website and this one for past exams.

Preparatory courses are expensive but there they tell you exactly what you need to know. I did one and it helped me so much, my mistake was deciding to go through with the exam only around 2-3 months ago and starting preparations then. I never had Biology and Chemistry before, while I last had Math and Physics at this level 4 years ago (one year should be plenty enough though). Price is quite steep as you said, AKAD is by far the cheapest option with around 2k but it's self-study. The other two on the website are in a classroom and with a private tutor respectively, but both are an order of magnitude more expensive for an entire year.

Honestly if you're comfortable with internet strangers we could get a beer or something (won't take it personally if not :p) and I'll tell you in more detail. I'm in the process of figuring out what to do next year too and I have to fit in military somewhere as well so we're in similar shoes.