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I would recommend doing the Bone Voyage quest and grinding combat at the ammonite crabs on Fossil Island, it's probably the most AFK friendly combat training until base 70 combat stats, which is when you should start either slayer or NMZ. PC is an alright way to train I suppose, and you need to do it eventually for void armor but that's not going to be useful for you until way later in your RS journey. I would not recommend rushing defence to a higher level than strength just to get 20k experience from a quest, at the end of the day 20k is a drop in the bucket compared to the exp you will be getting. Strength should ideally be considerably higher than attack and defence, although at the lower levels you may want to keep them consistent to be able to wield the higher tier weapons and armor.

Thanks a lot @Beefytron and @Butcher baby!!