Least labour intensive is evaporst without a doubt. Brim the tank and leave it. I left my DT3 tank for three weeks. Emptied and cleaned out,filled again with the same Evaporust but through a filter and left it for another week. Emptied and cleaned out then stills round for a few hours with 4ltr of clean evaporust. Emptied and let it dry without swilling . That was two years ago and it's still clean and never had anything in it . 
I have also used electrolysis which is highly effective but you have to take care with your set up. Cheaper if you have a charger already. The only other thing you need is distilled water and bicarbonate of soda . Switch on and leave it for a couple of days. Watch in amazement at the Black rusty crap bubbling out. Satisfaction in deed.

Vinegar worked  good for me. I left it in the tank 3-4 days.  I added screws and nuts, and shook it. I would use small chains next time the small parts were to remove., Stan