the opposition would be able to speak about the PM without the PM being able to directly respond

In republics where the president is head of the executive branch, the leader of the opposition / minority leader whatever might talk about the president without the president being present or able to defend himself. But, the head of the party in that chamber often defends the president or their policies.

Another thing to consider is that the PM being in the house of commons effectively elevates the leader of the opposition to being a near equal to the PM. If the PM weren’t even there, would anybody pay attention to what the leader of the opposition was saying? This is one reason why it’s probably a better system if the PM is in the house and does have to face the opposition. I think other systems suffer a lot from the lack of a “question period” where the leader has to face questions from some kind of opposition. Even if it often becomes a circus, it’s better than just tame press conferences. But, it might be better for the PM if they could basically elevate themselves to a head of state position and not just some “first among equals” in the house.

There are problems with both systems. The flaws of a system with a president who is the head of the executive branch and doesn’t have to deal with direct opposition are extremely visible right now in the US. But, at least in that system you theoretically have checks and balances between judiciary, legislature and executive. Clearly, the method those checks and balances are implemented in the US is completely broken, but it may be better in other countries like France, I don’t know.

OTOH, the system where the lead lawmaker is also the head of state has problems, especially when the senate isn’t doing its role to slow down the government and push back on bad decisions. There have been times in Canada where the PM can easily pass any law they want because they’re the head executive, head legislator and the senate just falls in line. IMO that situation has the potential to be even worse than what’s happening in the US right now.

For the moment, I definitely want Canada to keep doing the traditional thing and have a PM who’s an MP and does all the traditional PM things. Now isn’t a good time to experiment. But, it would be interesting to have a game of some kind where people could experiment with different systems and see if there’s one that works better. I also think someone should “red team” the existing laws and procedures for Canada’s government and spot weird things that are just done based on tradition so we can consider closing loopholes before someone exploits them, the way Trump is exploiting loopholes and things that aren’t fully locked down in the US.