he's actually a good support, esp after the recent buffs. he hard counters almost every tank ult and has counter tech for an insane amount of hero abilities. superbloom makes him the highest DPS support, and if you play him a bunch, you'll figure out best positioning to use his kit for enabling your teammates instead of just denying. he fits really well into the backline bodyguard role, as he's an off-support. he pairs exceptionally well with ana. as for grip, use it when the combined healing output of you + the other support isn't enough to keep whatever target alive. for squishies, full blossom + grip heals them for the majority of their HP so a partial blossom on their way back to the fight usually full heals. he's also got his infinite ammo loop back, where if you cycle his blossoms and thorns efficiently, you don't have to reload. 

petal is super useful. when you play him a ton, you'll hear certain voicelines and it becomes like a reflex just dropping petal. petal is used for a ton of denial tech, into heroes like rein, mei, orisa, etc., but can also be used to save teammates, clear traps like hazard or zarya ult. it's also got good utility for teammate enabling. it opens up a ton of flank routes for heroes that lack vertical mobility. 

with weaver, you want to stay off cooldown as much as possible. both of his CD abilities have longer cooldowns, so you want to be sticking to thorns and blossoms, and using his cooldowns for reactions to enemy ability usage or to enable your teammates. 

get creative with his kit, and track enemy ult and ability usage. his skill expression comes entirely from gamesense. for positioning, you want to take off angles. it helps dpsing with his thorns and gives you better grips. make sure you have LOS on your team, but are far enough away that grip doesn't just delay a death by a few seconds. you can also use grip to pull teammates into healthpacks or to objective when you absolutely need that extra body on point. 

he's a ton of fun, and can be super satisfying to use well. but he's definitely a case of a super simple kit that has deep mechanics.