Either seperating the queues or not showing in progress status.

TBH, I'd be in favour of either.

Seperating the queues won't cause the fresh queues to be longer because it's a natural balancing act: Fewer fresh runs because tanks are in 'In Progress' queue = fewer runs for people to join in progress = In progress queues take longer = Tanks less willing to rage quit because In progress queues take a long time.

If the 'In progress queue isn't as attractive to tanks because the queue is so long, more will join fresh parties.

And as for not showing the 'In progress' status of a queue... I don't really see any possible downsides.