You go back to that Vikings game in 18 where Goff was outstanding. Iirc, Cousins said to him at end of the game that it was the best QB performance he'd seen since he entered the league. Deep throws to Cooks, a killer deep seam throw to Woods...

The regression started with the Chicago game that year. And prior to that the Lions game, when Patricia seemed to spook Goff by loading the line with pressure. If I remember right, Chicago did this in spades the following week? (my memory might be out here).

Since then, what's changed in personnel?

Gurley was still a transcendent talent in that Vikes game. Not just as a runner but as a receiver...

Cooks and the deep ball. As you say. Even the year prior Goff had Watkins to hit on the bomb. I can't remember the last time Goff even threw a deep pass to a WR this season.

Do the field has condensed, there's no longer a great RB outlet in the passing game. And Ds know to follow the pressure pattern on Goff, that saw him bomb in the Superbowl.

Jourdan Rodrigue on The Athletic has written that McVay deliberately shortened up the Rams' passing game this off-season so Goff would throw less deeper balls and so less turnovers would result - sadly that hasn't followed.

It all feels way too bunched.

It's overly simplistic to say this, yes, but sometimes coaches can overthink things - and if you don't throw the ball into the endzone you can't expect to have touchdown catches.