I hate lean to’s. They aren’t as cheap as you think, you end up with a tunnel of a shed on many occasions, they are faffing about to bolt on, cutting holes in sheets etc. I prefer another shed spaced away gives a sheltered space in between and keep ventilation etc good.
we have seen more bother by spoiling ewes earlier leads to more problems as you end up with obese ewes that you have to put more feed into to keep them on their feet which makes them fatter in a vicious circle then the push their guts out.
I just wait to see what happens if weather turns like...
As above some hay if only to eek grass out as something dry in their bellies to keep them warm.
The way lambs have been fleshing off grass here it is plenty good enough for ewes.
I would drop crustalyx in just before you scan at Christmas in your situation so none go down with stress.