1. That is a pretty huge assumption. However I somehow did forget you just get to choose levels at level up. I don't fuckin know how.

2. This is super pedantic but considering everything relies on specifics, you just killed your train by saying "by crushing the mote". If the mote is destroyed then the spell disappears and you do not regain spell slot. A creature has to use its action to cast the spell for it to do so.

3. That being said, all of this relies pretty heavily on you taking this rest within an hours travel from the BBEG. The motes themselves only last an hour. So you can have a ton of familiars up but the beads that they have to activate? Those have a timer. The vast majority of situations will simply not allow you to be that close to the BBEG and have enough time that he doesn't notice people chilling and preparing to explode his life.

4. It would still not be instant casting. Each familiar, in combat, rolls its own individual initiative and gains its own turn. So you would swarm the initiative with different familiars with spells, sure, but the likelihood of instant casting is essentially 0%. Not entirely, but the odds are so insanely low that they would all go at the same time at the start of initiative.

5. It requires an action to dismiss a familiar so you cannot cast the spell and then dismiss them for unbreakable concentration.

I mean it works in theory, kinda, but it would never work in practice.