Seems all good to me just confusing tho because of all the variables, I will likely do the easy one you mentioned at least to start out with.
My concern tho is that you mention profits are dropping and there must be a reason for that is it because more people catch on the extra competition reduces profits? As I am considering the viability of buying new cards just for this.
I have a gtx 1070 which the link you provided estimates 100gbp a month profit which seems nice enough, at those profit levels it then makes sense to buy extra gpus as they will be paid for within half a year.
Consider you said this is a low profit now and that you used to get higher, it sounds like you were making a killing from this earlier.