I get what you're saying, but you're way overestimating the survival rate, and also using the myth that people (and everything that sustains them) will just move northwards. Habitability is far more than just the temperature range. You mention tundra, and the first thing to consider is how long it takes to form soil that things can grow in from a place that has none. It's hundreds to thousands of years for just a small amount. Then there's the weather. Warmer Earth means volatile systems and changing dynamics. Who's to say there will be a temperate, stable area in the few places left to live? It's not even something we can reliably simulate, as we have no idea what we're heading into.
There's more, but no reason to get too doom and gloom. I just wanted to point out the biggest thing your whole idea is resting on isn't something that's likely.
Only during the Holocene period. We're very resourceful, but there are limitations. The breaking point may not be our own selves, but things we rely on like food and water and temperature control.
I mean, sure. We may find ways to adapt, new food sources, get lucky with the weather behavior. Anyone who says they're sure of the future is lying. It just seems very unlikely in a runaway hothouse Earth scenario that we can do well. Large animal forms didn't do well in the last heating event (the PETM) and ones that survived evolved into smaller forms to do so (large exotherms don't do well with heat removal). They evolved because the PETM took tens of thousands of years to ramp up. I'm sure you've seen the science reports about how current rate of heating is faster than anything we have geologic evidence of before.
The next heatwaves will give a glimpse into how well we can adapt. The western world has its technology and energy to rely on for a while, the rest of the world doesn't.