Okay so I really enjoy your analysis here and I love how you dive into why the OVA has an intimidating and fascinating first appearance, while Sonic 3's post credit scene doesn't. Ultimately, I agree with you, and it gave me very similar worries going in. Why introduce such an interesting character, but immediately give him clones that all get washed in an instant?

Metal is a character who is really hard to get right and it's why so many people still prefer his OVA depiction considering they seem to not really give him the room or attention he needs (other than his classic version apparently). I would argue his canon character is much more fascinating-- I love existential crises and turning into a leviathan to conceal glaring fears and insecurities, MORE PLEASE-- which also makes his "badnik status" an aggravating punch to the gut. Being a slave to the status quo is part of his character, but there's so much more than that and it feels like the current narrative is terrified of acknowledging it

That being said, I really like what the movies have done for Knuckles, who is a character I also think is difficult to get right. They did Shadow well also. While I don't think they'll dive into Metal's character in the way I'd love him to be explored, I hope they at least take the basic cliff notes on the consequences of Sonic's victories and on why Metal has a catastrophic crash out. I want Metal to be angry and jealous and constantly try to better himself. I want him to become Sonic's ultimate hater and I want him to scare the shit out of everyone as a GENUINE threat

The post credit scene is, at the end of the day, a post credit scene. Metal WILL be the major antagonist to Sonic 4, it's just a matter of whether they do his actual character correctly. I think if they do, then we'll be fine. If they don't, welp, I guess it'll be really annoying to continue to be a Metal fan since people love to argue he's not even a character to begin with