Welcome to hell!
a) did i bought some unintentional boosts so i cut through the game like heated katana through butter?
No. the single player campaign sucks. pure and simple. This is one of the problems with D3 in that the single player campaign is too easy.
There is end game content, but progression is patchy. its split into 3 bits - adventure mode, Nephelim rifts and Greater Rifts. Since you cant get the items you need to progress in the solo campaign, the endgame grind is where you will spend most of your time. there will come a point where you need to stop playing solo and are forced to play multiplayer, but multiplayer is just running around after the person who is the strongest out of the players in the game and hoping you don’t get stomped by elemental or arcane damage until you find the items you need to progress
Difficulty levels in D3 are meaningless until you get into Torment levels at which point they scale up faster than you do, so by the time you hit Torment 7 or 8 you will be taking on more of a challenge, but wont have the items to progress (see above)
b) is RoS as easy as vanilla game?
Yes. its got a bit of a better story, but the enemies are a bit more annoying as they have after effects on death.
c) was it always like that or they screwed the game during patches and seasons and never cared to repair it?
This is a much debated point. D3 promised much but didnt deliver on that promise. It has its fans, but comparatively speaking, the community is pretty fractured between people who prefer D2 and those who dont like D2.
D3 tried to be more WoW like in the way it introduced seasons and major patch revisions opening new areas, but ultimately, it doesnt have the long term appeal (at least not to me it doesnt! 0 I am a returning player from before Season 1 just because of the D4 announcement. i have been playing D2 since season 5)
d) i assume the campaing is same for every characters so apart from different kind of grinding there is no reason to start another character
you can, and you dont have to play through the campaign! there is adventure mode that will level you just as fast. When you start playing seasonal characters, you will create different character classes as the builds for the new season are usually developed on the PTR (Public Test Realm) where patches are playtested by the community, so new viable builds for greater rift runs are developed and then blizzard will nerf or buff the builds to make them a bit more playable before the patch gets a proper release.
e) i’ve spent 30€ on the game and i assume am gonna finish it tommorow, what should i do next in the game?
- Adventure mode to build up lots of crafting materials
- Nephelim rifts for grinding items
- Greater Rifts to be able to up your difficulty level.
- Rifts are where you grind for gear. they are 3-4 level dungeons filled with lots of monsters and a rift guardian. killing unique monsters (they glow yellow or blue) will drop a few pickup orbs that add to a bar shown on the right of the screen. full the bar and the guardian appears. kill him and the rift is complete, but a word of advice… if youre doing a Nephelim rift, dont just end the game when you kill the rift guardian, finish the level off as much as you can as you will gain more legendary items that way, especially early on.
- Greater rifts allow you to gain Gems with powerful end game effects that can be upgraded. you can only upgrade the gems in a greater rift
- Post level 70 you earn Paragon levels. these give you bonuses to your damage and abilities.
- Play until bored.
luckily season 19 is starting next week, so it will be a level playing field for a few months, but until D4 comes out, these issues are going to be the reason you stick with it or drop it in favour of something else.