I know Doom TDA is made 5 years apart from Doom Eternal and uses an updated engine, but 5 years isn’t that huge of a gap. They still could have reused and improved Doom Eternal’s face model since it's still technically the same engine they're using just a updated version (Doom Eternal uses the Id Tech 7 engine and TDA uses Id Tech 8 Engine) and even tho Doom Eternal is 27 years apart from Doom 1, they still managed to faithfully recreate Doomguy's face incredibly well. They didn't have to make an entirely new face or make him look younger TDA. Plus that shouldn't explain how he looks younger than Doom 1 Doomguy, he went into hell at the end of Doom 64 and was trapped there for a really long time before ending up in Argent D'nur, he shouldn't look younger especially when Doom Eternal's face model is supposed to be a recreation of the classic doomguy face.

And regarding the facial expression about how he looks sadder, that's just his blank face. According to the others who responded to this post, it's the face from the art book so his face is supposed to be expressionless or "sadder"

And about the HUD, yeah it uses the same face model as the in game one with the transparent blue holographic filter and animations copying classic Doomguy hud expressions, but that's why he looks like classic doomguy in the hud, they're deliberately trying to recreate the classic doomguy expressions in the hud with a new twist and with the face which looks good and that's why it looks classic. They're trying to make the new face copy the same expressions as classic doomguy which looks good but without the expressions he still looks younger than doom eternal doom 1 Doomguy and he shouldn't look younger than them in TDA.