First off, GA4 and Adobe (AA/CJA) are different programs, I doubt you will ever get a complete 1 to 1 equivalent. Adobe does not track all those extra "engagement" server calls by default (which is a good thing as that would eat into your server call allocation). So you are right, there is really no way to determine the "Exit" (unless the user clicked on an Exit Link which is tracked), if they just back out of the site or close the tab/window. There is also no time spent on such a page view... both Adobe and GA4 use timestamps between calls to calculate that... GA UA never took the time spent into consideration either, only the addition of their "engagement" events allowed them to do this, and only if they are turned on.
In theory, you could possibly create your own custom engagement metrics, but you would be potentially using more server calls than you have been allocated for, which could result in overage charges....
I don't have CJA, but I do have Adobe Analytics....
AA has a "Bounce Rate" which is defined as a "single server call" divided by "Entries"
However, we don't like that definition, we prefer to track bounces as only a single page viewed, so we have a custom metric of "Single Page Visits" / "Entries"
I don't think that either "Single Server Call" or "Single Page Visits"... but I believe you can create a segment for:
Session
Events equals 1
To replicate a single server call in a session....