@Ray072 

This is what I can tell you about current state your Deco mesh, based on screenshots you provided.

Having Deco M9 at 2 bars is OK. You will almost never get three bars, they must be really close to achieve that. Definitely not for Deco nodes on different floors. I have Satellite M9 Plus on second floor right above Main Deco on the first floor, both facing stairwell. Yet, even in that almost ideal setup my second floor Satellite M9 reports two bars.

So, consider any Deco M9 with two bars properly placed. I suggest you check from time to time "Show List" and if signal does not drop below two bars - nothing to worry about. 

I would also suggest to check if configuring "Loggerkamer" to connect to "Slaapkamer" will give better speeds for devices connected to "Loggerkamer." If not, restore current configuration.

Also, because your Satellite Deco may occasionally prefer to connect to Main Deco, I suggest to explicitely configure "Kantoor" to connect to "Slaapkamer".  Connect "Loggerkamer" to either "Slaapkamer" or to "Kantoor", based on results of previous test I suggested.

That leaves "Schuur". Does it still show weak signal? I thought after you configured it to connect "Woonkamer", which is nearby, you've got quite decent speeds from "Schuur". 

If "Schuur" still shows weak signal, please make screenshot of the top part of its info page and share it, example of what I need to see:

Other than that, it appears you have sufficient number of Deco nodes. If you still want to add more, I strongly suggest buying additional M9 Plus and not WiFi range extenders. Buying M9 Plus might be more expensive, but it will be better in a long run. You can configure new Deco nodes to connect to the mesh the way you want them to, and also you will have consistent WiFi mesh management through Deco app.