You've come up with a good summary of the events in time.

It is clear that whatever caused the ring structure happened before either of the two large impacts in the area. The ring structure has been struck by numerous secondaries from the later impacts. Those secondary impacts are obvious, but they do not seem to have significantly eroded the ring structure. This -- along with the generally well-preserved nature of the ring as a whole -- indicates that it is formed of highly competent material that is not easily eroded.

Some circular formations on Mars that look a lot like impact structures do show some nascent ring-like rims. But most do not. For example, the vast majority of the older craters in the Southern Highlands show no raised rim at all. Most fresh craters everywhere show typical raised rims that resemble those in the THEMIS image. These rims, however, are not likely to erode to rings.

All over Acidalia (and also found elsewhere) there are huge ring structures that appear to have resulted from impact since they have extensive ejecta blankets. Their rims are low rounded rings like the small crater in the THEMIS example. Their interiors appear to be filled with something that was once fluid, possibly icy rock or a very runny lava. The ejecta appears to have been fluid as well. In some cases, the fluid that once filled the craters appears to have overflowed the rims. In most cases, however, whatever filled the interior of these craters sloshed around and dried inside the crater while the ejecta appears to be composed of some other material.

I am aware of no small ring formations that show any signs of impact origin. (That doesn't mean there aren't any -- my eyes got real tired long before I looked at all the available images!!) Instead all the ones I've seen actually resemble one another strongly, most very similar in size for one thing, many filled with something that is eroding, sometimes in striking patterns.

While it is possible that all these ring formations originated as impact craters, and they have all been subjected to flooding of one kind or another, and they have all eroded in essentially the same way, I think it is just as possible that these formations originated through some other process altogether and that they are not impact structures.

I think we need to questions assumptions about these and many other aspects of Mars.

Cheers, BFelix