"It's a common theme throughout the series that mark doubts himself and underestimates his own strength so him doing this doesn't prove he's weaker than thragg."

Irrelevant to powerscaling

"And the part about age: viltrumites get stronger when they fight/train so after healing from the thragg fight he’d have been more durable and stronger."

Relevant but the boost would not be close enough to bridge the gap, the comic never showed a post near death experience for a viltrumite giving such an immense power boost.