Professional / formal version of "sitting pretty"

Solution 1:

From Collins dictionary's definition of be sitting pretty:

If you are sitting pretty, you are in a very favourable situation.
[informal]
If we'd let them buy it for a quarter of a million, we'd be sitting pretty by now.

Alternatives to sitting pretty (and the sentence) are then as follows:

This leaves Plan A in a good position, and [it] remains the only option that keeps Goal A firmly on the table.

This leaves Plan A in good shape, and [it] remains the only option that keeps Goal A firmly on the table.

This leaves Plan A doing well, and the only option that keeps Goal A firmly on the table.

This leaves Plan A well positioned to keep Goal A firmly on the table.

Solution 2:

pretty TFD adv

fairly, rather, quite, kind of (informal), somewhat, moderately, reasonably

Thus, my sense points to reasonable as a non-idiomatic and more formal alternative in any context:

As in:

This leave X reasonable and ...