I've kind of tried gradual retreat, but as its a tiny box room it's not really having much effect.
I don't know what Gradual Withdrawal means to you, but to my way of doing GW, this seems like an odd sentence. Why would room size matter?
Gradual Withdrawal means staying with the child until asleep. Always giving baby enough comfort, reassurance and presence. But with a view that over time (slow and gradual - so over several months) small changes are made with a view to eventually reaching independent sleeping.
So now you breastfeed to sleep. Aim for unlatching and replacing with a dummy as soon as baby falls asleep, but continue holding baby in BF position until asleep. Then aim for the point where you can place baby in BR position but give dummy instead, cuddle until asleep.
This ^ would be progress, not breastfeeding to sleep.
Then tiny changes to get baby into the cot. So start settling baby to sleep in a cradle hold but not breastfeeding position. Then try doing it standing up. Then start putting baby down sooner - say if a 10 minute cuddle usually means baby is fast asleep and you can place in cot, try putting baby down just before fully asleep and then leaning over and into the cot to settle baby. Stay until asleep.
Each change involves still staying with baby until asleep. And no change is made until baby is happy with the previous change. Keep the changes tiny, almost unnoticeable.
Aim towards settling baby to sleep in the cot and staying with baby while going to sleep. Maybe a reassuring hand on baby's chest/back. Then just standing by the cot until asleep. Then waiting a step away from the cot, staying until asleep. Then facing away from cot. Then in doorway. etc.
A small room is actually good for Gradual Withdrawal, certainly the latter stages when the weekly changes involve withdrawing from the room. There will be fewer small changes needed until you are waiting outside the door for baby to go to sleep.