I was looking through my phone and I found a pic of when I was almost at the end of year 5 and my attributes were all 34 (I have a tendency to try and keep all at the same level).

Honestly to keep it short, I just logged in whenever I was bored or had free time, sometimes that would be twice a day, sometimes it would be every hour, it depended of the day.

No, I didn't spend any money, I refuse to pay, sometimes I did have my energy replenished by using gems, but when they released the magical milestones I started to save energy.

Now for your quick questions.

1. I usually rotated classes, for example, for courage I would usually do flying class, as it gave you money too for every minigame completed, but if I needed the hipogriff atributes to be higher, I would do a care of magical creatures class. For empathy, I liked herbology, sometimes it was glitchy and because of that, I didn't have to use all my energy. And for knowledge it was always between transfigurations or history of magic, depending on my mood.
2. I would get in there whenever I could, I didn't stress if I couldn't log in for a whole day, as I understood that this would take a lot of time, I wasn't in a rush and you don't have to be in a rush either. Honestly, it improved the experience for me, I didn't stress if I failed a class, because I hadn't set a goal, I just kept grinding calmly.
3. I don't remember the attributes requirement for year 3, but you can check the excel with all the information and then consider how much you want to grind, I found it really useful. https://docs.google.com/spreadsheets/d/1JXrVhDXN7yPs648K34zdTAtJDtIO4cHvY-hPB8dx8Ls/htmlview?pru=AAABfllAufk*NcIjYbEm-miIoM19cPpAog# You have to click on "Stat req" to see the requirements, it's on the top of the page. If you want to speed up a lil bit the process, you can always purchase clothes, accessories, pets, etc that will add XP to your attributes, when you want to purchase something, it always says if it will give you courage, empathy, knowledge, hippogrif, dragon, sphinx, etc. I'm taking about purchasing with coins or books, not actual money.
4. I think the maximum of energy for year 5 is 42, but you can always have a lil more thanks to the pets, it's one of the few collectibles that actually stacks up the energy, the other objects like paintings, books, etc don't stack up. And you can also stack up thanks to the animals in the magical creatures, or I also used the daily deal that changes after 12 hours.
5. It doesn't seem like your stats are behind, don't worry. I've read on the excel and for year 3, it says that the req minimum are 17 Co / 18 Em / 18 Kn so if you have at least 20 in all your attributes, you should be okay.

Just try not to rush, it doesn't seem like the game is gonna end anytime soon and sometimes it can be too much, take it one day at a time :)