They fell for a number of reasons - any one of which they could have shrugged off, but they all came at once.
Well... "at once" over the course of several hundred years.
loyalty to the empire strained by imposed religious reformation to some strange new monotheistic cult
That strained the Senate far more than the general populace, who were quite happy accepting yet one more god.
and then all that during a succession crisis which left the empire fragmented and unable to muster up a unified response.
If you're going to say the succession crisis caused the collapse in the latter years of the empire, you need to explain why the succession crisis didn't cause the same problems during the Crisis of the Third Century.
you can't find a single year and declare the empire ceased to exist here.
September 4, 476 was the official end of the Western Roman Empire. The Eastern Roman Empire lasted 1,000 years after that, when it fell to the Ottomans.
But back to September 4, 476. Odoacer turfs out Romulus Augustulus and sends the robes etc. to Emperor Zeno, saying that they were no longer required. Now, granted the western empire was in ruins at this point in time, but this date is the accepted date for the end of the empire.
... empire built on constant expansion ran out of new land to invade for tribute
That's not even remotely true of the latter empire. The later republic was certainly built upon constant expansion; however the Varian Disaster in 9 AD put a northern border that the empire didn't grow beyond. Trajan had the greatest territory expansion, this was mainly to the east; and his reign ended in 117 AD; long before 476 or even the crisis of the third century. Hadrian consolidated the new frontiers but didn't push past them.
There's no one factor that lead to the collapse, and the collapse itsself was a slow process
That's not quite true. The prime factors are the rising of the Sasanian Empire, a collapse in tax revenue, and loss of the growing areas in Northern Africa.
The rise of the Sasanian Empire caused the empire to move northern border troops to the east. The now porous northern border allowed the Germanic tribes to start to invade; the Germanic tribes themselves were being pushed out of their lands by the Huns. The Germanic tribes moved along Gaul and Spain, and crossed into Africa, capturing the the fertile regions there. Meanwhile, other Germanic tribes at first started ransacking cities and towns, but soon discovered it was much easier to offer to defend the towns and rule. These Roman towns and cities then directed their tax revenue to the Germanic rulers, depriving Rome of much-needed funds. As the funds for the armies declined, so did the armies. Roman tax collectors were not only unwelcome, but forced out of these new Germanic areas.
The Western and Eastern emperors agreed that recapturing North Africa was a prime concern, and mounted probably the largest military force ever seen to do just that. But before the fleet could sail, Atilla the Hun started his 10 year rampage, diverting Roman attention to this new menace.
Following Atilla's death, there simply wasn't the money to raise an army to retake North Africa, and the Western Empire effectively ceased functioning around 410 AD, with the empire formally coming to an end on September 4, 476 when Odoacer deposed Romulus Augustulus and declared himself ruler of Italy.