@Generaltwig 

They provide you with WiFi mesh solution: this modem/router and two satellites run as WiFi mesh. I checked their specs: they are similar to Deco mesh. They are WiFi6 which is enough for gigabit Internet link. Satellites have two Ethernet ports - one of them can be used to connect Satellite to modem/router with Ethernet cable. You should check that part, but I believe it is true. As such, you don't really need to add Deco mesh to it. 

I would strongly recommend to go with your ISP mesh NETCOMM NF20MESH WiFi mesh solution. If my ISP offered that when I needed to improve WiFi coverage at my house, I would not have bought Deco. Going with ISP systems will ensure better level of support from your ISP in case something goes wrong. 

It is possible you may need more than two Satellites, but you can buy add-on Satellite separately, according to what I see on the Internet. It appears, there are NS-01 and NS-02 Satellite models, you'll have to check by yourself what is difference between them. Perhaps, your ISP offers additional Satellites - rent or buy more directly from ISP if necessary.

Having said that, I am going to give you answers on your questions, because most of them do not really depend on WiFi mesh hardware you will be using. If you have very strong opinion against your ISP's NETCOMM NF20MESH, let me know and in another post I'll give you Deco mesh hardware models and configuration specifics optimal for your house.

• Overall: Does this plan look good? Are there any improvements?

The plan looks good overall, but these are improvements I am going to suggest:

1) You are taking two Ethernet cables to Master Bedroom. I suggest you lay just one cable to there from the Office and add gigabit switch in Master Bedroom to connect two devices (PC3 and TV) through it;

2) You can use ISP Router Ethernet ports in the Office, that router has 4 ports. PC1 and PC2 will connect directly to ISP Router, third port will be taken by the switch and you can have remaining one to connect Ethernet cable from Main Bedroom to it;

3) On your diagram you have three Ethernet cables going from the switch in the Office through the wall to TV room. Perhaps, you could place that switch in TV room and put just one Ethernet cable through the wall between Office and TV room.

To cover future home networking needs you don't anticipate now, I would suggest also putting Ethernet ports in Bedroom1, Bedroom2 and Living Room. With Ethernet cables from these ports going all the way to the switch. 

At a completely bare minimum you should have Ethernet cable going to the Living Room. I suspect WiFi coverage in that room may not be great and if it is not, this is where you'll need to add one more Satellite.

• Router/Modem: Can I use a standard ISP router/modem?

As I strongly recommended, you should use NETCOMM NF20MESH with its Satellites. If you insist on Deco, you should still use ISP Router NETCOMM.

• Switch:   Recommended switches by TPLink for Backhaul on Mesh setup seem to be : TP-Link Switch TL-SX1008 V3 or TL-SG108 V8. Are either ok? Is the cheaper TL-SG108 v8 going to slow me down?

Any of these switches is OK, as well as other TP-Link gigabit switches. I still suggest TP-Link switch, even if you go with NETCOMM NF20MESH. TP-Link switches support Deco mesh Ethernet communication protocols between Satellites, and I suspect these protocols are utilized by other mesh brands. Just in case, get switch from the place with return policy. 

It may also be a good idea to contact your ISP tech support with the question what gigabit switches they recommend to use to connect Satellites through them by Ethernet cables to ISP Router NETCOMM NF20MESH.

• DEVICES: The image shows 10 devices. I see the switches generally only have 8. 

With what I proposed, you will have free Ethernet ports on 8-port gigabit switch. Actually, two free ports if I counted right. Use them for Ethernet cable coming from Living Room and Ethernet cable coming from either Bedroom1 or Bedroom2.

There are also 16-port Ethernet switches such as TP-Link 16 Port Gigabit Ethernet Network Switch (TL-SG116), but that might be an overkill. These are larger and if you could manage with 8-port switch, no need to spend extra.

If you want to cover for future cabling expansions, then do indeed go with 16-port switch, but choose one with passive cooling (a.k.a fanless). You would not want extra noise coming from switch fan in either Office or TV room. Same as with 8-port switch, check with your ISP what switches they do recommend. If they don't have preference, go with TP-Link.

• DECO PLACEMENT: Do they look to be in good positions? Can I place them inside cupboards? Or rather attached to the wall?

Satellites look to be in good position, and same positions should be for both Deco and NETCOMM NF20MESH Satellites. You may need one more Satellite in the Living Room and possibly Satellite(s) in Bedroom1/2. If you have Ethernet cables going there from the switch as I suggested, you can easily add Satellites where WiFi coverage is not as good as you would like it to be.

You should never place Satellites inside cupboards. Here is recommendation from NETCOMM documentation: "Do not restrict airflow around the device.The device is air cooled and may overheat where airflow has been restricted. Always allow a minimum of 5cm clearance around all sides and the top of the device. Do not cover, do not put in an enclosed space, do not put under or behind large items of furniture.Your device may become warm during normal use."

That applies to Deco, too.

• CABLE: What kind of ethernet cable do you recommend? Cat-6?

My house is wired with Cat-5E, but I have cables that are no longer than 20m. I am getting solid 900Mbps from them, and this is as good as it gets on gigabit networking equipment. 

Cat-6 should be OK. One really big caveat: in last few years there had been increase in Ethernet cables sold that are not up to specs. You should suspect even those that are sold from reputable sellers. I would recommend to test each cable before laying it through the house. Do not trust cable tester devices, they may lie to you. Only practical test can do, unless you want to risk redoing cabling.

Practical test: take the cable and connect it to ISP Router that is on gigabit Internet. Connect another end of that cable to Satellite. Use second port of Satellite to connect PC to it. Run speed test on a PC: speeds close to gigabit (or whatever max you could get from ISP Internet link) are good, very close to 100Mbps means Satellite can't negotiate gigabit speed over that cable and downgrades speed to 100Mbps, which means cable is not up to specs. Use NF20MESH app to verify that Satellite uses Ethernet to connect to ISP Router, because if Satellite does not use Ethernet - that cable is definitely defective.

Only after you tested that cable in this way it will be safe to lay it through the house.