Using "to, co" as a relative pronoun

Hello,

I was wondering if "to, co" as a relative pronoun (meaning "which", "what" or "that which") always declines regularly?

For example, if I wanted to say:

I have finished what I was doing.

I think this would be:

Skończyłem to, co robiłem.

If this is right, then would "to" become "tego" in the negative?

Nie skończyłem tego, co robiłem.

And if "robić" changes to "szukać", does "co" become "czego"?

"Znaleźłem to, czego szukałem." OR
"Nie znaleźłem tego, czego szukałem."

Thanks in advance!

yes in proper Polish grammar those two decline regularly in situations you have presented - they actually should always - lower and lower level of education in Poland however makes things as 'znalazłem to, co szukałem' and 'co szukasz?" instead of czego szukasz common - actually I am not without fault here

Thank you very much, Gumishu.