Not so sure about the rift attacks being so early in the game. Given the range on the units and the massive hp (especially on hard mode), you’re tanking early/mid game progression. The rift spawns are going around killing anything that gets close to them, meaning they’ll advance the goblin campaign whether the player is ready for it or not.
I’d recommend not having them spawn until 20 days after you’ve promoted someone to a time mage. Maybe have a quest line before they appear where a portal opens up and a Time Mage steps through. A couple quests of them requesting gathered/crafted goods and they offer to teach a level appropriate Herbalist their secrets. Time Mage and herbalist hop into the portal, herbalist instantly reappears (cause Time magic), and then after someone becomes a Time Mage the portal starts to grow bigger. Twenty days later, rift monsters start to appear until you destroy the portal. It’s kinda similar to the ACE quests to unlock bone carving for masons with an added destroy mission at the end. You may also wanna consider taking the existing rift mobs, making your own version, and trimming their stats down a bit so they’re more appropriate for the stage of the game you want them to appear.