All good suggestions here and I agree with most all of them. I'll add my thoughts, but a lot of them have been said already.
I think I would isolate each loop with isolation pins in the center rail of each cross over switch. Use one handle of the Z4K for each loop. I think you could then run a DCS train on one and a conv. train on the other, just don't cross over between loops. You could also use the fixed channels of the TIU and operate conventional by using the Z4K handles (or possibly the Z4K add on device?)
I would probably just quarter each loop to create blocks (easy and even that way), again isolation pins in center rail between each quarter block. Then run a pair of wires to the center of each block, length doesn't matter. I think the idea is to have a send and receive wire to each block of somewhat similar length for xmit & rec - TIU to track as DCS is 2 way communication.
Many folks say to connect the two outer rails with a jumper wire, but others leave one outer rail not connected to common for operating accessories, signals, etc. I have a smaller layout not a lot different than yours and I have one outer rail with no common connection and it works just fine, but every layout is different. If you have a problem I suppose you could always add jumpers as needed. If you do use just one outer rail, make sure you use the same one throughout on each loop.
Then I would isolate the center rail for each siding/spur where I thought I would park a train. I would install a switch in the wire to the center rail to toggle power to the siding/spur. It's good to turn off DCS engines when not being used as the clock keep ticking on them if powered.
As for the circle and wyes, I am not sure anything needs to be done there other than conform to the 10-12 joint rule and possibly create an extra block there if needed, just like the rest of the blocks. If you plan to use it as a siding to park a train then some thought on isolation pin placement and adding a switch for center rail power might be needed.
Personally I am a big fan of the MTH terminal blocks and OGR wire. These things make it all pretty easy to wire up a layout for DCS. I think OGR was out of wire a while back and I am not sure they are going to restock or what, but it was twisted pair #14 & #16, all copper and very good wire. I suppose a replacement could be found somewhere?
For switch operation I used thermostat wire from Home Depot. It's fairly inexpensive and available in 2, 3, 5, 7 & maybe more conductors, 18 ga. I used mostly 2 & 3 conductor to wire my track switches. Only drawback is it's solid wire, but even so it is still pretty easy to work with. FOr me it worked out better than stranded because I didn't need any crimp on connectors, just wrapped it around the switch terminal screws and tightened the screw. This would work fine for accessories, lights, etc. as well.
I am sure I probably forgot something, but much has been covered above so someone has probably gotten it somewhere here.
This is pretty much what I did on my layout and it has worked flawlessly for a few years now. I have not gotten one DCS error since I got it set up, other than self inflicted ones that is. Mineis a little different as I use PH-180s for power and also have a Legacy system. Mine is also a one table layout (6' x 16') and not an around the room one as yours is, but we are pretty close here on many things.
Sorry, but as I said earlier I don't have a Z4K or the added control device for it so I can't offer any help (or would that be more confusion?) there.
I have a somewhat accurate picture of my layout & wiring around here somewhere, I'll post it when found. It is similar and might give you an idea of something similar to yours.