What would be the best way to train this Tuna God?...

Lst is always the best and one of the only ways to increase yield . Just and topping only work up to a point and really work only if you veg a long while longer . The plant is small though you should expect twice that size indoors . Growth rate is controlled by VPD which is the relationship between temperature and humidity but there's only so much you can do about that outdoors ... Might as well bend it and pray for it. I don't know if you have plenty of time before flowering based on your sunlight but hopefully it's enough time for the plant to acclimatise a tiny bit at least. What you really need is 6 degrees less and some more humidity but you're outdoors nothing and no nutrients can really help you except maybe try to spray the kelp on the leafs (same dilution ratio as in the feed) twice a week , it contains something called auxin which is plant growth hormone ... Hope this helps 🚀