Sounds like quite a project you have going on there!-)
let me try to give you some answers. In reverse order
you didn't mention if your running Linux or Windows. But thats not realy to important.
Solomining is more profitable at that size (~100TB), but it can be frustrating at times.
Your read speed will be perfectly ok an that machine!-) My preferred "weapon of choice" is jminer. manly because I can run it on the integrated (Intel) GPU and it gives me a read speed of ~20sec for 80TB. So I'll imaging you could use any old Graphics Card to do the mining. Blagos miner for CPU/Win or creepMiner for ...well, everything I think, will also do just fine. As longe as you have those resources free while plotting.
So you bought Seagate's SMR Drives! Well then!-) They are a pain to plott but perfectly fine for mining! More on that later..
Don't worry to much! the new Pocc pools 100-0 / 50-50 / 0-100-pool.burst.cryptoguru.org work just fine for example. You'll be going solo anyway after you have plotted up to size.
Sounds about right. Me: oldish I7/7cores/13000 nonces/min. Balance is key! More on that l.....
Ok; The limiting factor for plotting IS disk write speed most of the time! Because of that its usually better to wright to a few discs in parallel. Also, there's no need to optimize if your using Xplotter for Win or oc_obup for linux. They all ready are optimized.
But, you bought SMR drives wich all the "cheaper" and Seagate ones are. Those drives dont like the way we plot or, more precisely, how we write to them! they are fast at the beginning but once the buffer is full the write speed drops down to almost nothing. like 5Mb/sec!
The best way we come up with is, to plott to some sort of SSD first and than do an sequential write to the SMR disc. That way the head does not have to jump around or something!-)
For doing that you will have to write a script, adapt this one , do something with this or use Splotter(not recommend).
Next thing you want do is to find a balance between the cores your using, the amout of Ram and your disk write speed (it should be a SSD by now!-)
Xplotter does everything asynchronous. Meaning it uses half of the Ram you've given him to create nonces and writes the other halve to the disk at the same time. For the fastest overall speed you never wont any of those two operations to wait for the other! Thats what I mean by finding a balance. With Xplotter you can stop the process change "amount of cores" (-t) and "Ram" (-mem) but keep everything else the same and it will continue from where you've stopped when restarted.
Plotting needs time! Unfortunately! Me, with my oldish highend(ish) gaming Rig, never made more then 1-1.5 TB a Day.
My sugestion would be to buy yourself a couple of Intel Opten P900 SSD's and give them to me when you done!-)
Hope that mumbo-jumbo helped some..