One issue I noticed besides the mentioned team recognition is I feel like this could be mistaken for a Demoman vest at a glance since it makes the front of his torso and shirt flaps dark grey.

I understand it sounds silly because the default colors of Soldier and Demo help differentiate them and you did keep the shoulders and back untouched. The problem is I feel like with enough creativity it can be used in loadouts to make it tougher to differentiate at a glance, especially from front angles.

Other vests like the Patriot's Pouches and Flakcatcher still keep a chunk of Soldier's colors intact, like keeping his chest or shirt flaps the same color from all angles.

The only other issue I have is there’s clipping with the chest plate on the belt when he slouches, which is a common posture for him.

I do like the look of the metal and paint patterns. The shoulder plate and belt also work well. The rest of the armor's design just needs some adjustments to work alongside Soldier more.

Covers up too much team color

if you're that upset you can check it on loadout.tf

Yet to be proven.

Nope, there's a strap at the back

i like how he never shows the back that will be completely grey

It's still weird how unfinished those medieval soldier hats are without a proper set like this. You'd figure valve would've added something like this by now

Yeah, this one gonna get the whole "team recognition" type of talk. Should have been easy one look at to understand that.
Well made, its just that covering majority of the model in grey is not good idea in the first place especially when the small areas of team paint on them are minimal.

Flipping the gray and team paint colors would be better.

Finally something to go with his conquistador helmet! Keep up the great work!