We use the term "immunity" but we should remember that there is a difference between the presence of active antibodies and the presence of immune "memory cells". The latter hang around long-term, even if the former disappear. So at a minimum if one does develop infection they will recover far sooner and with better outcomes. And likely will reach a far lower peak viral load which might bring down transmission.

That is not well established.

It's likely enough, but we don't gain long term immunity to every virus that we successfully clear.

It is well established.

https://en.wikipedia.org/wiki/Memory_T_cell

You are conflating active circulating antibodies with the memory T cells. Which is literally what my entire comment was about.

Of course, there's other confounds. Like the somewhat recent discovery that we end up with antigen experience / "memory" of diseases that we've never contracted. (Perhaps from viral fragments in the environment??)