"of that" vs "of which"? word "chief" can refer to more than one item in a set?

Since the last time I messaged you, there have been a number of new developments in my life, chief of which being graduating from college and getting my own place.

Can I say "chief of that being"? Or "chief thereof being"?

What's the difference between "of that" and "of which"?

And is chief only used to refer to one item in a set, or can it be used to refer to two like I did above (graduating college, and getting own place)?

No, you cannot use chief of that being. Although that and which are interchangeable in many contexts, after a preposition is one in which they are not.

[This is one of the principal reasons why that that is no longer regarded by grammarians as a relative pronoun, but as the same subordinator as in He said that they had gone. The argument is that the underlying form of the relative clause is which that, but in modern English at least one of these must be deleted.]

Chief thereof being is possible, but very literary.

To my ear, using chief like that does lead the listener to expect a single item: in that context, I read "graduating from college and getting my own place" as a single item. If the two parts were not closely connected, I don't think it would work.

If you use that you are confusing the subject of the sentence, and using something that refers to a singular object rather than plural objects.

There has been more than one development. If you refer back to that, you need to use them, not that:

Since the last time I messaged you, there have been a number of new developments in my life, chief of them being graduating from college and getting my own place.

Note that neither was nor were follows chief of them. If one or the other did, you would be explicitly stating you are referring to one development or two developments.

Although chief normally refers to a single thing, it doesn't always have to. It's similar to best, where two or more items can, jointly, be considered the best:

John and Sally were the best students in their class.

With chief:

[Merriam-Webster]
2 : of greatest importance or influence
    // the chief reasons
    // their chief accomplishment

The first example sentence has multiple reasons that are chief. The second has a single accomplishment that is chief.

In the sentence from the question, it's possible that graduating from college and getting my own place is considered as a single conceptual item. For instance, depending on how it's viewed, washing and drying my clothes can be one overall action or two discrete actions.