if youre using spellbook as malz you should be running or tping down to bot to ult xayah as soon as that shit is off cooldown. to win in bot with malz isn't too hard, you shove the wave and either roam down to bot to ult them, or if you dont have lane prio, you shove that shit in as hard as you can when the other mid is roaming. i think malz is fine to play until you get to an elo where people start buying a lot of quicksilvers, which isn't until much later on, although I feel the hextech item for malz isn't necessary. get more damage with deathcap or void instead of that.
also what the fuck lol dodge if you see your team chimping out or not taking the right summoners. losing 3/10 elo per dodge is nothing, doesn't affect mmr at all.
buy a pink ward every back and put it somewhere near your lane, either in one of the river bushes so you can blind their laner vision on you, or put it back around your jungle if your t1 turret is dead. once the game gets past 20+ min you can pink ward baron and clear it, and have your team set up around it but not do it. with any luck, the other team will be forced to facecheck and you can just ult someone as malz as the rest of your team beats the fuck out of them.
idk if this is considered optimal or not, but sometimes i do like rylais on malz for the sole reason he can be played more as a defender/peeler if you have someone on your team popping off. save your ult for someone that can jump in and kill your damage dealers; when they go in, ult them, E them, disrupt their team with Qs, kite back, etc. malz is really good for defensive peeling as well as holding down anyone to kill.
try to always watch the map for plays to make with teleport, one good tp play to bot and you can get your team 2 or more kills, putting you in a great advantage. try to tell your team's bot lane to ward the enemy's side of the brush, as it makes it easy to tp in if they overextend and engage your team.