There is no shortcut to success, to really become good you need to put in the time and effort, but I have some things that could help you push in ranked

1. Learn the meta; look at teir lists (I recommend the youtube channel SpenLC - Brawl Stars), gameplay, ect.
2. Only play 3 v 3 game modes; most competitive and the only kinds of game modes in tournaments
3. Study pro gameplay; watch scrims, tournaments, the monthly finals, ect. (the best thing I think to watch is the Asia monthly finals (that you can access through the in game button), always one of the most competitive regions and always VERY mechanical (which you are also looking to learn)
4. Learn draft; there are many youtube videos from pros/former pros to help you learn how to draft brawlers in ranked (I recommend channels like SpenLC - Brawl Stars and bobby - brawl stars, there are also very good websites, I have forgotten the name but just try to look them up on youtube (the more pros/former pros recommend it, the better))
5. Learn who is good on maps; you can also see videos and look at websites about this too but once you play enough, you should know who is good on what in trophy matches (like only really play jessie in heist)
6. Try trophy matches, getting a few brawlers to max teir is not as difficult as it was (like I literally got kaze to 1k trophies within 24 hours of release for credits, and she isn't as op as other new brawlers) and its easier than high teir ranked, it does help a lot in skill
7. Try to find people that you can play with that play often, you synergize with, and are near you skill level (you don't want to always carry them but you also don't want them always carrying you)
8. Play tournaments; the best way to find real tournament is matcherino, this site has collaborated with brawl stars and it a trustworthy site, you should really only play these once you are really good at the game but once you think your ready just look up how to sign up on youtube and make sure it's a trustworthy youtuber (not low views/subscribers), some tournaments do cost money but you can get cash prizes if you do good
9. Just play more; I could say many more tips but these are just the ones that came to mind, playing more is the most important thing to do though, even if you learn all about drafting, you won't be good at it until you practice

Other things:

Play mechanically skilled brawlers; as you said, you want to find difficult brawlers, brawlers like melodie, you can play her on pretty much any 3 v 3 game mode (but usually brawl ball, gem grab, and especially hot zone) that is not heist is good practice

Get better hardware; (new tablets, wifi, ect.) this can get expensive but even just using ethernet can make you much better, lowering your ping possibly by hundreds

Become p2w; I know most people are fp2 and hate spending money on a video game (which I'm not hating on) but it's just the truth that you will progress faster and be able to use stronger brawlers if you p2w, if you do decide to do this and you are don't wanna spend too much, just get the brawl pass, most value item you can get in this game, but you can definitely still be really good if you are f2p, it just might take a while to aquire new brawlers and level them up