R’hlloric resurrection doesn’t require a third party sacrifice, but Thoros states that he loses a little of his own ‘life force’ every time he performs the ritual. Do you think the blow will be greater for Melisandre since she is so old and possibly frailer than she seems?

That’s not really what Thoros says:

“Thoros, how many times have you brought me back now?”

The red priest bowed his head. “It is R’hllor who brings you back, my lord. The Lord of Light. I am only his instrument.”

“How many times?” Lord Beric insisted.

“Six,” Thoros said reluctantly. “And each time is harder. You have grown reckless, my lord. Is death so very sweet?“ 

Thoros is saying that it is harder to bring Beric back from the dead, not that he himself is losing life force in the attempt.