's shows either possession, or when the following word i.g. is/us/... is abbreviated. Therefore, in this case, 's can only be used if you're talking about something that belongs to "thank", which makes no sense at all; nor if you're saying "thank is" which does not make sense, either.
The s at the end of the word "thanks" is just a plural s and adding any apostrophe before that is just an error. Native speakers in each language may happen to make mistakes.
Word origin:
Date: 1300-1400 Origin: Plural of thank 'gratefulness' (11-17
centuries), from Old English thanc 'thought, gratefulness'
Says the subscription-only LDOCE.
To make your appreciation/gratefulness in a more formal way, you could say:
Thank you so much for your assistance.
Update:
As mentioned in the comment, if "thank" is being used as a verb, s would be added to make it third person singular:
She always thanks her mother for the meals she cooks.
However, there's another note that can be a common mistake among learners or non-native speakers who happen to use the verb form with that third person singular s for all persons; so instead of saying
"Thank God" or "Thank heavens" or "Thank goodness",
they'd say:
"Thanks God", etc.
Here, the full sentence would be: "I thank God for ...", and obviously, "I" is first person, rather than the third.
