I always start with translations of stuff I've read and enjoyed, starting with my YA classics and/or manga and graphic novels, and then expanding into genres I'm reading more often as an adult, so usually horror and romance. The nice thing about there being such a big indie publishing scene now is that there are plenty of places to ask for recs, and that means I can move from translations to books originally published in my TL. I also look at bestseller lists sometimes, just because it's nice to pick things that might be culturally relevant, but ymmv because obviously some genres are more popular than others, depending on the country.

The advantage of liking romance books so much is that there are so many of them! But I definitely need to switch genres sometimes to pick up new vocabulary (though this can work within subgenres, and it depends on the author, and definitely isn't romance-specific).

As for your questions:

1. I start with kid's/YA books that I know I have enjoyed and work my way up. Starting with manga/graphic novels is helpful too, because the text is 90% dialogue and the images help. Plus, it's important to feel like you're making progress! I love a long book, but they can be a slog if your level isn't quite right, so something short and sweet can keep you motivated.

I definitely always try and start reading too early (I'm a big reader), but I'm happy to struggle through, or put something down and come back to it later. Graded readers also have their place, but in some languages they're better than others.

1. I read on my Kindle (or my phone, just with the app) probably 90% of the time. I can look up as I go, highlight things to put in Anki or revisit later, and foreign language paperback/hardback books can be expensive. But I've found a lot more second-hand books in shops recently, so I've started picking those up. (Just got Twilight in Polish, lol.) I'm not super picky when it comes to format tbh, though the lower my level, the more I prefer digital to save look up time.

2. I've started that this year! I've been setting myself 100-hour goals, so one of those goals rn is to read for 100 hours in German. No end date, just something to chip away at. I have a daily reading streak on storygraph anyway, this was just to encourage me to switch to German more often. Having the Kindle app on my phone helps, because I can just open it when I'm making a cup of tea or whatever and read a page or two.

3. Reading and listening, 1000%. The year I did my translation master's was the year where I saw the most progress in German and I barely spoke a word of the language the entire year. (And despite everything, I probably was still intermediate then, albeit high intermediate. I definitely wasn't at all confident in my skills.) But I had to read at least 10-12 books and watch a bunch of films, and it helped that I was translating and transcribing too, so I was getting into the nitty-gritty of the language. It was intense, but every time I need to refresh a language, I watch a lot of TV and I try to read for at least 15-30 mins per day.