@Salts 

Hey and Welcome

Hopefully be able to answer some of your queries!

1.

Is there a way to force the Decos to use a specific channel or at the very least force the APs to rescan the network and try selecting a better channel?

Yes.  In the Deco app there is a option for Network Optimisation, this will rescan the airspace and change channel if appropiate.

2.

Can I get some insight on how the Deco determines which channels to select? Is the channel selected based on the signal congestion from just the primary unit? or all units?

Pretty sure its all units, the network as a whole.   Only know this as on my friends setup it has interference on 1 of the 3 nodes (channel 36) and his Deco always avoids 36 when optimised.   

However, in terms of optimisation remember this is a no win situation.  Your neighbours ISP routers are likely to randomly change channels following reboots / schedule etc so the channel that is best today, may be completely congested tomorrow as your neighbours also make the hop to it.  This is a common issue and therefore you may find that the Network Optimisation is always reporting congestion when you run it.  

Honestly, if its not causing you performance issues then don't get too caught up on this as its not something you can control 

  @Philbert 

Unfortunately, that is not true. I have run optimiser six times and it always chooses channel 36 and states wi fi channel is congestion.

I wish I could change it on my own.

I'm open to other suggestions 

  @Rx2000 I have a similar situation, the deco xe75 in access point mode, works flawesly almost in every room of my house. The problem I have is in the kitchen, my house is on a corner and the kitchen is the closest room to the adjacent apartments, where both the 2,4 and 5ghz bands have the same channel as the strongest neighbouring wifi,(some stronger than mine). Which makes it impossible to listen to music/ radio on a smart speaker(google nest audio).

A solution would be to be able to manually set channel or that when using the option(of the app) to optimize the network, to use the mobile to collect data on the status of the wifi in that room.

Now i think it collects them from the nodes/router and does not solve the problem.
Can anybody help or should I return the router for a ASUS XT8?