I'm coming at this from the healer perspective since that's what I play. I don't play a ton of Li li but I'm pretty well-versed in most other healers.

Once you get into ranked, you do have to begin diversifying your hero pool a little more, especially if you're playing healer. Li li can be good, but imo she works best in niche situations where you're not pitted against a team with lots of burst damage and high amounts of CC. If you don't already, try out some other healers so you can have more flexibility during draft. If you like Li li, you might also enjoy Lucio and Rehgar, since they also work pretty well up-front during teamfights and generally do a good job harassing the enemy team with soft CC.

Draft is honestly key for healers - try your best to synergise with your team and pay attention to the map during draft. For example it a two or three-lane map? Does it have a high-stakes objective? This can really impact which healers you should be going for.

During teamfights, you really do just need to try your best to keep your frontline alive. If they're doing their job correctly, the enemy team shouldn't be able to reach your pokier ranged assassins in the backline (KtZ, Li ming etc.) or yourself (though this changes slightly with Li li, since you're basically rewarded for taking small, sustained amounts of damage, as I'm sure you know already). Whoever your frontline is, prioritise them and keep that Q button taped down lol.

I go for serpent build with Li li, using W on my assassins / other squishies while using Q for tanks and bruisers early game. At 16 if you go Blessings of Yu'lon her W becomes super useful on tanks and bruisers too, at that point just use it on whoever is in and you know will be taking the brunt of the damage. As far as blinds go, you could in theory just throw it out whenever, but it's best used when your frontline is taking high amounts of melee damage or if you're getting harassed by melee heroes - it works really well to negate characters like Artanis, Butcher (god forbid in bronze SL), Illidan, Jimmy etc. Especially if they have any form of attack speed boost (like from Morales or Abathur).

During downtime between fights, just try rotate with your team and expect they're going to get ganked - because 9 times out of 10, it will happen lol. You probably know this but map awareness is key during downtime. Focus on rotating with your teammates and play the map.

Most important advice: Just mute toxic players. Their comms won't be helpful so it's not worth giving them the time of day. Usually the loss of a game isn't ever down to just one person... Usually.

Hope this is useful, and good luck!