Well i had no problem with leveling up while doing Quests but i never played till max level on the KR one.
Doing main story quests lets you earn more EXP with your stamina bar, but doing side quests can earn you titles and benefits you would miss out. Depends how you want to play the game either speedrunning to end content or being a completionist and have everything unlocked.
Exactly, I'm with Katsukai.
It pretty much depends on your playstyle and how fast you want everything unlocked/completed.
Personally speaking, I'd forfeit reaching cap fast for having everything unlocked as soon as possible.
But at the same time, do everything at your own pace. I've found myself burntout sometimes when doing something solely for the reward and not just to have fun.
But I'm also kinda curious hove Gameforge will address it. They might take some quests out since purely grinding isn't the most favoured activity to us western consumers.
I did the sub quests a lot for the free material.
Did anyone else have trouble juggling for inventory space by completing all of the sub-quests? That seems to be an issue I've been having.
Joining a guild would help a lot with inventory management as they have their own item storage. There is good guide about guilds if you wonder how it works in-game. Guild Guide
You must do at least blue and yellow quest, you can skip green quest until you find yourself inside ruin fortress.
Blue quest give alot of exp, so if you want to rush to cap the better way is to do all blue quest and some green quest of ruin fortress
In my opinion its a game about following the quests for stories. So in that matter I would suggest you to try it out 
The best option would be doing main and sub quests at the same time for more exp and material. Green quests (daily quests) is better to do in ruin fortress for more exp and in the new area called grass cover camp that will be released alongside the class advancments for farming s coins.
