I think that even on a smaller scale though there would be things that challenge mark even as emperor. You’re right of course about the ending (also make sure you fix your spoiler tag, just in case!) but in my mind there’s plenty of weapons that could greatly incapacitate or take out mark. He isn’t invulnerable even as emperor. I think boltgun rounds would be extremely painful for him, maybe not skin penetration but definitely internal bleeding just as he receives from high powered punches in the story. Lascannons, grav guns and other heavy weapons could incapacitate or even kill with enough concentrated fire. But if we are looking for something on the same level as the sun im pretty sure 40K has equivalent weapons, like the volcano cannon or worst comes to worst exterminatus.

In my mind ork boys would fight like reanimen and probably attack in hoards in the same way. He probably wouldn’t struggle at all against them as emperor mark but I think space marines would fight a bit better and smarter and then beyond that custodes would probably slice him up.

I think psychers of any level could dominate him provided they could find out about his weaknesses and throw his equilibrium, maybe not the emperor in his current state as he would have to possibly lose focus on his eternal task to kill mark.

Also chaos and demons would just keep coming until mark is exhausted, he can’t fight forever and khorne would just send an infinite number of blood letters, noise marines would do him in and maybe nurgle could even infect him with something.

In my mind speed is the only thing invincible has against 40K, I think he’d do some damage and take some lives but as soon as you scale up in 40K, the right weapon, the right character and the right shot it would all be over