Zerg mid-game involves preparing for mass production, trying to harass your opponent and reduce their harvesting. Mid-game I say is about 9-12 mins.
You can transition, and the best yet is to perfect infestor use. Well, maybe not anymore, but still I find infestors to be the best crowd control unit there is.
You also need to keep in-mind that at least, for every 5 minutes of the game, you have taken a new expansion. and added +1 hatch and queen per 2 expansions. In fact, you can get up to 5 hatch total for two expansions for the mid to late phase.
Probably, it's either infestors, and broodlords. These things are necessary for a zerg.
Also, upgrades by 7:00. There's a lot to perfect and enjoy trying to get right. All it takes is familiarity of the phases and available options.
Lastly, I advice against suiciding units. Especially lings. These are very costly during the early game and mid game since they eat a lot of larva.