There is a FB group that provides step-by-step instructions on how to file a Writ of Mandamus. I also consulted with my lawyer, who sent me a pricing list. We agreed that I would complete the first step on my own, and if step two becomes necessary, he will take over the case.
He advised me to contact my MP, request ATIP notes, and send a demand letter to IRCC. I had already completed all of these actions before receiving his advice. In my initial demand letter, I gave IRCC 30 days to finalize my citizenship application. Following my lawyer’s recommendation, I sent a second letter extending the deadline by another 30 days. That deadline has now passed.
Interestingly, I received a ghost update after 15 months, and I’m not sure if it was triggered by these actions. Based on my experience, I recommend first sending a demand letter to IRCC via webform and regular mail.