I was gonna give the ususal Lord of the Mysteries reccomednation but then I read "not so plot heavy" which kind of makes it impossible to reccomend since it won't fit your criteria, also makes all my favourite webnovels impossible to reccomend since they are all very plot heavy. Also "rpg system" in your tag which means you prefer character focused isekai power fantasies which I really am not a fan off. However I can still think of two high quality webnovels that fit your requirements. First one which isn't on this site for somereason: 'Shadow slave'. There is a pretty big focus on him hiding his strength especially in the beggining and the worldbuilding is incredibly well done. It has every criteria your looking for and most importantly it is well written, the level of quality contained in it is almost unheard off in the webnovel world. The second novel which also isn't on this site for somereason is 'supreme magus'. It is the most popular fantasy novel on webnovel.com and even though its been a while since I read chapters of it I am certain it fits all of your criteria while still being of acceptable quality. The characters are all fleshed out and the MC hides his strength, supreme magus fits you're criteria the most out of the two but I still highly reccomedn reading them both. They both have the exact things your looking for. I reckon that the opening chapters of both books are so gripping that there's no need for me to pitch them to you. Especially the prologue chapter of supreme magus. Edit: they both have some romance forgot to mention that but since you didn't specifically mention you don't want romance, I assume they are still ok for you. they both have no yuri or yaoi. Also the romance isn't the main focus