Python 3.8 (pre-release),  158   144  143 bytes

-14 bytes by Unrelated String.
-1 byte by emanresu A.

t=lambda s:(r:=len(s)//2)and[l.center(r*2+1)for l in[s[r]]+[f'/{" "*i}\\'for i in range(1,r,2)]]+[*map(' '.join,zip(t(s[:r]),t(s[r+1:])))]or[s]

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Python 3.8 (pre-release),  158   144   143  142 bytes

-14 bytes by Unrelated String.
-2 bytes by emanresu A.

t=lambda s:(r:=len(s)//2)and[l.center(r-~r)for l in[s[r]]+[f'/{" "*i}\\'for i in range(1,r,2)]]+[*map(' '.join,zip(t(s[:r]),t(s[r+1:])))]or[s]

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Python 3.8 (pre-release), 158 bytes

t=lambda s:(r:=len(s)//2)and[l.center(r*2+1)for l in[s[r]]+['/'+' '*i+'\\'for i in range(1,r,2)]]+[i+' '+j for i,j in zip(t(s[:r]),t(s[r+1:]))]if s[1:]else[s]

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Python 3.8 (pre-release),  158   144  143 bytes

-14 bytes by Unrelated String.
-1 byte by emanresu A.

t=lambda s:(r:=len(s)//2)and[l.center(r*2+1)for l in[s[r]]+[f'/{" "*i}\\'for i in range(1,r,2)]]+[*map(' '.join,zip(t(s[:r]),t(s[r+1:])))]or[s]

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