A child can get a suprising amount of comfort from the enclosed feeling the cot bars provide. A bed is quite exposed and much less 'nest like'.
Personally I find the transition from cot to bed is much smoother if done at a time when parents feel the child is emotionally and behaviourally ready, rather than when forced before the child is ready. So if possible, I'd recommend going back to a cot and teaching him not to climb. If that is not possible, you could try to recreate the cot-like feeling by using a bed tent and/or bed guard.
A drastic alternate solution would be the family floor bed - a double mattress on the floor.
In terms of self settling, the path of least resistance is all very well, but teaches the child very little. Teaching a child to sleep independently doesn't need to be distressing, but it is short-term hard work so wouldn't qualify as the path of least resistance.
(having said that, when you think about it, 9 months of parental hard work to get baby to sleep independently verses several years of dealing with a child who cant sleep independently - makes you question which option actually is "easiest")
So the gentlest approach would be simple acceptance. By around school age it is reasonable to expect that a child can generate his own comfort and wont need external means to gain the comfort needed to feel secure enough to sleep. You could simply accept that your DS will need your help to get to sleep for a long time yet and in your ethos of taking the path of least resistance, just work out how you can all have the most sleep and the least stress by accepting this will be the way of things.
Or you could decide to start working on self settling. From what you describe it sounds like Gradual Withdrawal would work for you. It is gentle and slow, so is not a quick fix.
GW involves making small, gradual changes towards independent sleeping. Ideally you want to encourage him to bond with something that he can use as a comforter in place of his hands on your boobs. Maybe you could start shoving a muslin square in your cleavage and encourage him to thumb that with his hands buried down there. Then slowly, gradually, have the muslin square less buried in your cleavage until it is the muslin that gives the comfort rather than his hands in your boobs.
Then encourage lying down with the comforter, with you snuggling next to him. When he's OK with that, start turning away from him slightly, but still stay until asleep. Then turned away but slightly away from the bed, stay until asleep. Then further from the bed. And so on.