I would agree upon the uniformity of shield shapes and designs, for units level.
curved scuta (rectangular and oval), flat rectangular, hexagonal and oval shields however speak a different language.
The gladius in the early empire was certainly a standardized weapon.
The highest functional uniformity is evidenced for the infantry helmet.
but functional uniformity is firstly natural for an army with increasing diversity and grade of "combined arms" tactics.
Uniformity in modern terms however, would be something very different.
roman wargaming miniatures are made very standardized,
rectangular scutum, metal stripe armour, imperial gallic helmet and that's it.
similar for auxiliaries. This is a modern concept of uniformity that I would reject straightout for anything before 1700
but we are indeed diverting here
whatever the reason for the shields to look brighter/newer, be it repair or frequent replacement (one shield per battle is of course only the case for units actually involved in CC, which was rarely the case), this appears to be a fact - the wargamer seems to be good advised to let them look newer.
If we really want to have a productive discussion about warfare before black powder, we should do it maybe in another thread