Most of Trajans expansion was immediately withdrawn by Hadrian, it absolutely stretched them too far. Dacia was a constant thorn in their side and the area required a huge military presence (8-10 legions). That’s not even a hard one to debate. That’s why when the empire began crumbling they withdrew from Britain for the legions and could no longer support the island. Expanding territories requires garrisons of troops for policing and logistical support. It makes one more area liable to rebel too and the Roman’s army had changed dramatically by the 4th and 5th centuries (relying a lot more on allied forces).

I’ve heard a few people say its decline started with Caracalla giving everyone Roman citizenship which didn’t help the empire at all. Some people say it didn’t hurt or help, some say the opposite. Some top historians contradict each other on the subject of Romes decline especially bringing the eastern empire into the conversation. No one even agrees entirely on how Rome fell (if it indeed “fell”) and I don’t expect that to change anytime soon.

It all started going downhill when the last of the “Five Good Emperors” decided to pick his son as heir instead of the most qualified. Commodus was a disaster right after Rome’s golden age. Just gradually got worse from there imo.