Political Memes

Rome built itself on the back of it’s military with a strong economy, as the economy became stable, the military eventually began to dwindle with it until Rome could no longer defend itself.

Other way around. As long as the economy of Rome was stable, the Empire was prosperous. The Empire's prosperity nosedived because of the constant unrest of the Crisis of the Third Century. Turns out when you spend a great deal of time killing and plundering each other in civil wars, there are less men left over to kill invading barbarians. Who knew!?

The economy had been suffering for a while. Overspending on the military and foreign wars was part of it, but not all of it.

The military and foreign wars were probably the least objectionable thing the treasury went towards. Just about every half-decent Emperor that came in would inevitably look at the extravagant court costs of their not-even-half-decent predecessor and say "Fuck me, is THAT where our money is going?"

The wealthy began to retreat to the far frontier and set up independent fiefdoms to avoid paying taxes.

Not even to the far frontier, or by force, sadly. They just... stopped paying. The political power of the Emperor no longer depended on the loyalty of the class of nationally ambitious politicians, nor on the exceptional loyalty of the (then-conscripted and life-term) military, nor on the Roman people, but on local magnates who were more than happy to leverage both their small size (making the Imperial apparatus crushing them not worth the cost) and their outsized importance for all it was worth.

As Rome stopped expanding the ability to replace slaves had vanished and fewer men were willing to fight.

Gotta point out that those two things aren't related. The decline of slavery after the peak in the 1st century AD had few serious effects on the Empire. Fewer men were willing to fight because years of constant civil war, the complete breakdown of the society you were supposed to be defending, the propagation of regional loyalties, and then capped with a 'divine' autocrat prosecuting sectarian rivals who relies on conscription and lifelong sentences to a much-less professional and meritocratic military is... well, it's a bit of a downer.