"only so that" is correct and usable in written English.
It is used to express a purpose or reason for an action. Example: She changed her job only so that she could make more money.
I was alive only so that I could witness myself dying".
If he gave the ball up, it was only so that he might get it back.
Only so that when it is done, we can truly say we did it.
He craved success only so that he could provide for his family and help others.
He clings to life only so that he can find Isolde and take her with him.
He said he was lying only so that his family wouldn't find out about the affair.
Ice top of cake only, so that it resembles a frothy pint of Guinness.
Bad news: that's only so that he can be sacrificed on top of a pyramid.
Judge Childers said they sought custody only so that they would not be deported.
He imagined a world where colleges exist only so that prospective employers can figure out whom they want to hire.
I said, 'Yes, I am for détente, but only so that all cards are on the table.
