That's a great project! Here are some sources that I think would be quite interesting to look at, but, as you said, they are only a narrow selection of viewpoints and are certainly not representative of the full range of contemporary opinions!

It is perhaps worth starting with Augustine, the late fourth/early fifth-century North Africa Church Father. He preached a number of sermons in the aftermath of the Sack of Rome by the Visigoths in 410, all trying to calm the citizens of Carthage when the news reached them - in his view, the Sack of Rome, devastating though it might be and which had caused some people (pagans?) to blame Christianity for the empire's woes, did not signify the end of the empire or God's displeasure. This can be partially reflected in writings of Ambrose of Milan a bit earlier when he was caught up in a dispute over pagans - these letters are translated in the excellent Translated Texts for Historians series published by Liverpool University Press. The alternative, pagan perspective is perhaps best summed up by Zosimus, an Eastern Roman pagan bureaucrat from the early sixth century who literally blamed Constantine and Christianity for the all the problems the empire faced since then. His New History is helpfully translated in 1982 by R. T. Ridley and an interesting recent study of this debate in the sixth century is Shane Bjornlie's chapter in The Life and Legacy of Constantine: Traditions through the Ages (2016).

Anyway, Augustine's views were then developed further in his magnum opus, The City of God, which essentially tried to disentangle Christianity from Rome's problems. A neat summary of Augustine's thought-process can be found in Peter Brown's Augustine of Hippo: A Biography (1967/2000); the new edition adds some interesting material at the end that helps to contextualise Augustine's world a bit more, but the core material on the 410s remain the same. Augustine's friend, Orosius, took this further and wrote a History Against the Pagans (also conveniently translated by LUP in 2010), which, as you'd expect, argued that the empire was doing perfectly fine and had recovered after the Sack. A particularly striking piece of rhetoric can be seen here:

He [Athaulf, king of the Visigoths] was accustomed to relate that at first he earnestly had wanted to obliterate the name of Rome and make the Romans’ land the Goths’ empire in both word and deed, so that there would have been, to put it in everyday speech, a Gothia where there had once been Romania and that he, Athaulf, would now be what Augustus Caesar had once been. But when, after long experience, he had proved to himself that, because of their wild barbarism, the Goths were completely unable to obey the law, and because he believed it wrong to deprive a state of laws (without which a state is not a state at all), he chose at least to seek for himself the glory of having restored and extended the Roman Empire by the might of his Goths and, since he could not be her supplanter, to be remembered by posterity as the author of Rome’s renewal.

This is of course only a speech put into Athaulf's mouth by Orosius and should be not taken as something that actually happened, but it does represent how some people in the fifth century thought about the prospects of the empire (and in turn, what kind of views Orosius' opponents had...).

This is one perspective, but other writers were more pointed in their assessment of the empire's problems. Take for example Ammianus Marcellinus, a pagan historian writing under Theodosius I, who bewailed the corruption and decadence of the Roman elite of his age, and which contributed, for him, to the disasters leading up to the Battle of Adrianople. Another interesting perspective is provided by Priscus, an Eastern Roman ambassador to the court of Attila the Hun who wrote down/made up a conversation he had with a Roman deserter, in which the former citizen justified his life choices this way:

He considered his new life among the Scythians better than his old life among the Romans, and the reasons he gave were as follows: "After war the Scythians live in inactivity, enjoying what they have got, and not at all, or very little, harassed. The Romans, on the other hand, are in the first place very liable to perish in war, as they have to rest their hopes of safety on others, and are not allowed, on account of their tyrants to use arms. And those who use them are injured by the cowardice of their generals, who cannot support the conduct of war. But the condition of the subjects in time of peace is far more grievous than the evils of war, for the exaction of the taxes is very severe, and unprincipled men inflict injuries on others, because the laws are practically not valid against all classes. A transgressor who belongs to the wealthy classes is not punished for his injustice, while a poor man, who does not understand business, undergoes the legal penalty, that is if he does not depart this life before the trial, so long is the course of lawsuits protracted, and so much money is expended on them. The climax of the misery is to have to pay in order to obtain justice. For no one will give a court to the injured man unless he pay a sum of money to the judge and the judge's clerks."

This cannot be taken as an accurate record of what happened, if it happened at all, but it does reflect, I think, how some people viewed the late Roman state, as one that was unjust and incapable of ruling its diminished empire. A similar, but more religious perspective was presented by Salvian in his On the Government of God, written after 439:

Meanwhile the poor are being robbed, widows groan, orphans are trodden down, so that many, even persons of good birth, who have enjoyed a liberal education, seek refuge with the enemy […]. They seek among the barbarians the Roman mercy, since they cannot endure the barbarous mercilessness they find among the Romans.

A slightly different and a much harder text to understand is Gildas' On the Ruin and Conquest of Britain, in which the early sixth-century author (?) blamed all the Romano-British' failings in the fifth century on their lack of piety. This, and the sources above, are all well-studied and you should be able to find old translations of these texts quite easily online.

Finally, you mentioned that you don't want to get into the historiographical mess about this topic, but in some ways I think you need to if you are to discuss the 'fall' of Rome, as a substantial number of scholars would consider the 'fall' to be a later construction. There are plenty of sources whining about the collapse of the Western Roman state, but it is perhaps worth thinking about how people nevertheless repeatedly tried to preserve it. Rutilius Namatianus for example wrote a very jolly poem about his return to Gaul after serving the imperial government in 416, whilst imperial ambitions to restore its power never ceased, best seen in the naval expedition against Vandal North Africa in 468, a mere 8 years before the 'end'. Indeed, for contemporaries, the Roman world did not end in 476, though also remember that for some people it really did, see for example J. Harries, Sidonius Apollinaris and the Fall of Rome, A.D. 407-485 (1994). In Italy, it has been argued by Jonathan Arnold in his Theoderic and the Roman Imperial Restoration (2014) that contemporaries applauded the Ostrogothic ruler as a new emperor. In Gaul, Ian Wood has written persuasively about how the Burgundians did not primarily see themselves as kings, but as Masters of Soldiers (a traditional Roman title) of the emperor in Constantinople. For these people, the Roman order had evidently endured, rather than shattered during the breakdown of centralised government.

As I said, this is a huge topic and this answer definitely did not cover all the material (or even only the most important ones), but I tried to provide a few different (and hopefully interesting) perspectives. Let me know if you have any questions!