Mastering a Chmapion

i just had a ranked game and i select katarina. as i wanted to lock it in i cklicked on kennen, who i never palyed before and locked. in a promotion game i didnt want to leave and tried my luck. all i knew about kenne was what i lernd whlie playing against him. demn and i chrushed my lane opponen..

1 ) now my question: how important is it to master a champ`? or is it maybe better to play denecnt a bunch of champs that can counter otehr champs?

2. is every champion woth to put effort in? a garen needs not manny game to "master" him.. while a riven need tons of practice.. i have palyed aroudn 500 katarina games... but i not sure im getting better with her.. 62.1% win rate 11.6/6.8/ kda is not bad but where woud i be if i played 500 games with different champs?

3. also master one champs leads u to a lack of other lanes... when im foced to play adc...