ssTTsSTtRrriinInnnnNNNIiinngg
Gaia, 25 bytes

ṇ\+†ṅ\⟨)₌¤:~+4ṛ⟨ṛ₌¤⟩ₓ\⟩¦$


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Thanks to Kevin Cruijssen for pointing out 2 bugs!

ṇ\&#9;&#9;&#9;&#9;| delete the last character from the input
  +†&#9;&#9;&#9;&#9;| push the input again and concatenate together, so for instance
&#9;&#9;&#9;&#9;| 'abc' 'bc' becomes ['ab' 'bc' 'c']
    ṅ\&#9;&#9;&#9;&#9;| delete the last element
       ⟨       &#9;&#9;⟩¦&#9;| for each of the elements, do:
&#9;)₌&#9;&#9;&#9;| take the first character and push again
&#9;  ¤&#9;&#9;&#9;| swap
&#9;   :&#9;&#9;&#9;| dup
&#9;    ~&#9;&#9;&#9;| swap case
&#9;     +&#9;&#9;&#9;| concatenate strings
&#9;      4ṛ&#9;&#9;| select a random integer from [1..5]
&#9;        ⟨    ⟩ₓ&#9;&#9;| and repeat that many times
&#9;&#9; ṛ₌¤&#9;&#9;| select a random character from the string
&#9;&#9;      \ &#9;| clean up stack
&#9;&#9;&#9;   $&#9;| convert to string

Note that 4ṛ is because ṛ is implemented for an integer z as python's random.randint(1,z+1), which returns an integer N such that 1&lt;=N&lt;=z+1.