Correct on watercooling the 1080ti and obviously the monoblock. It would be nice to get the ram also unless that isn't recommended.
Some people watercool their ram though it really does nothing performance wise. I would suggest using two pumps for sure if you decide to. Each water block you add to the loop you create a restriction point. Dual pumps will ensure you have the head space to get proper flow. It also adds redundancy to your loop. In the event of a pump failure you are less likely to burn up $7-8k worth of equipment.
I was worried that there would be too much heat for a single loop. If you think that a single loop will allow a solid overclock I'll probably just go with that for this build. Using the 100w per 120.1 worth of rad that GTX mentioned I'd need a 480 rad?
This is a topic of much debate here and other PC forums. The general conclusion is that a single loop will run a few degrees warmer overall, maybe 2-3c. This is marginal at best and with a proper amount of radiator space a non-issue. I would say 480mm of rad space would be a minimum. You mentioned you plan to overclock the system so the TDP of your components will increase from stock listings. If you were to add an additional 120mm to that formula you should have enough room for a solid OC. So you're looking at 1x360mm & 1x 240mm radiators or 2x 360mm depending.
I don't know enough to talk intelligently about the bay reservoir...from what I've read it limits upgradability and would be more challenging for a beginner? (ref
https://www.ekwb.com/blog/pump-reservoir-combo-or-separate/).
I've only ever used the combos so I'll defer to those with more experience on this.
As far as pump/reservoir combo or not. I like the looks of separate a bit better. However...my initial thought would be combined would be better for space and less chance of me burning up the pump as a noob.
So long as the reservoir is located to feed into the pump and you don't run the pump dry you won't have an issue. Never place the pump before the res in the loop.
I adjusted the psu down to a 850 which still gives headroom. Price isn't much difference but as you said the 1250 was a bit of overkill for the build =)
850W should be plenty for this build. Many people don't realize the efficiency of the PSU is based on the load. If you are loading a titanium rated PSU at 20% it will be less efficient than a gold rated at 80%. Don't quote these numbers as they are just for an example. We could do the math between the two but over the course of a year we're really only talking about a few dollars (unless you're running multiple mining/folding rigs).