I'm glad that GOPers regret boycotting the House J6 investigation. That just indicates that even Republicans are realizing the televised hearings have been effective. But if you read their quotes, what they regret is Kevin McCarthey's decision last summer to refuse to allow any Republicans on the House panel (except the ones that Nancy Pelosi picked), which meant they couldn't get anyone on the panel to derail the investigation. That's what they regret. They don't want a more thorough investigation in to what happened, they regret their missed opportunity to throw a monkey wrench into the process so less would get investigated.
They also seem to have no regrets in blocking the effort to have a bipartisan commission (like the 9/11 Commission) investigate the incident. It was only after Mitch McConnell killed that legislation that the House formed its own committee to investigate January 6 on its own.