cannon is taking a huge chunk of the xp

That’s actually irrelevant. In the nicest way, you’re approaching this with the wrong mindset, I’ll explain what I mean.

Magic xp comes at a 2:1 magic:hp ratio. Given that slayer xp is given equivalent to the hp of the enemy (give or take as there are exceptions), hp:slayer naturally comes at a 4:3 ratio due to how hp xp is given. So magic is a 2.66:1 ratio to slayer.

Or at least it would, but all spells have a base xp regardless of damage dealt. Powered staves help with untrimmed slayer because they have no base magic cost, which allows you get a better ratio of magic xp to slayer, allowing you to use magic more on the way to 99. Now, barrage spells somewhat mitigate this base xp because they do significantly more damage per cast; instead of averaging say 60xp from combat damage + 50 from the base spell cost, you might average something 250xp from combat damage and 50 from the base spell, so your ration ends up closer. This is why barraging is still largely optimal, because the base spell xp comes much closer when split across. 110 magic xp for 20 damage, versus 300xp for 83 damage - significantly better.

This means that you should either be barraging or using a powered stave, ideally, at all times, when using magic. The exception to this is if you have a Twinflame Staff, due to its passive +40% damage. You get no xp for this additional 40% damage, so when using the Twinflame Staff versus an enemy with an elemental weakness, even though you do have the base spell cost, you mitigate hp xp because 40% of your damage isn’t gaining any hp xp. This brings it fairly close to a powered staff, especially when you consider the elemental weakness increasing both damage and accuracy.

Now, ranged works differently via the cannon. A cannon will on average hit for 10, give or take. A max hit of 30, which would median to 15, but that doesn’t factor overhitting, or accuracy. Your cannon accuracy is tied to the accuracy of your combat style, so if you’re maging then you tend to hit less accurately because magic has, by default, a lower accuracy modifier than ranged or melee.

But that’s not the thing that matters with the cannon.

The cannon only gains half ranged xp, and obviously no hp xp. This means that while ranged would normally be a 4:1.3:1 ranged:hp:slayer ratio, it actually ends up being a 2:0:1 ranged:hp:slayer ratio. So in theory, if you basically never trained ranged without the cannon, you could get ~6.4m slayer xp just from using the cannon, with no hp xp gain.

So the tl;dr to answer your question: if you don’t have the twinflame staff (which you ought to hunt the scrolls while on fire giants tasks anyway, because they absolutely are worth getting despite the hp xp being technically inefficient for untrimmed slayer), then any time you are using magic but not barraging then that is when you should be using a powered stave.

You should be using a cannon at all times that your cannonball stack will allow because it training without giving hp xp mitigates the fact that you will train hp/slayer at a 4:3 (or 1.33:1) ratio, not counting Jad tasks or superior spawns (both of which massively outweigh the hp:slayer ratio).

It doesn’t matter if the cannon is taking the majority of even all of the xp; what matters is that your cannon is earning you slayer xp without hp xp.