Alch in between swings while slaying. Pretty much the only time alching is useful, for multitasking. You've got fairly balanced combat stats, why specifically train strength? Training on controlled and powerslaying you'll still hit max melee before 99 slayer. An SS is inferior to a whip+defender, and that's the only realistic option (dharoking is subpar for slayer due to slayer helm bonuses, and a scimmy just plain sucks). Doing slayer without trying to mage tasks, and then barraging later would be faster. Only a few tasks are good for maging, and only one I can think of is actually faster than melee.