Rebreathers are not sustainable, they require a supply chain to be able to keep using them. The scale of the supply chains required to keep humanity alive in such a situation is far beyond what can be achieved through bunkers.

The only way we could survive such a situation is where the environment will become survivable within decades, which in evolutionary/geological timescales is a blink of an eye.

No one is building and preparing bunkers for surviving centuries to millennia, because it's not feasible.

Those bunkers would need sufficient access to new materials for indefinite use (particularly energy production). You could not produce a full supply chain to produce the necessary tech to replace aging components in the bunkers, from inside the bunkers, within a few decades. At which point, your motors, batteries, ICs, etc would all start to die, and with them, us.

If human population crashes too far, we will lose the majority of the skills, technologies and knowledge we've developed.

If we've fucked up the environment too much (like we're in the midst of doing), we're gonna die off with the rest of the planet.