@Pimpek 

This is good plan and yes, you do need WiFi mesh. That is the right replacement for separate networks of pre-mesh WiFi routers (went through that myself).

Recommendations:

If you have Ethernet cable between Living Room and Office 1, you should connect Main Deco with Deco 1 by Ethernet cable.

Better place for Deco 2 could be not in the Office 2 but in the Hallway.

It is not easy to tell without testing, if Deco 2 can reach Deco 1 and establish good WiFi link between them, which is a must for well functioning WiFi mesh. To test that, I would recommend to place standard WiFi router where Deco 1 will be, and take smartphone where Deco 2 will be. Using WiFi Network Analyzer app (WiFiman is my recommendation) measure WiFi signal strength between smartphone and WiFi router.

If signal strength is good in that test, Deco 2 should be able to reach Deco 1. 

You will need to configure Deco 2 connect to Deco 1 and not to Main Deco. This can be done in Deco app, but not all Deco models support that functionality. I'll recommend those that do.

With Internet link speeds of 750/250, WiFi 6 capable mesh will be sufficient. No need to spend extra on WiFi 6E or WiFi 7 mesh.

With all of the above considered, my recommendation would be 3-unit set of Deco X55 or Deco X50. These two Deco models are very similar in hardware specs and appearance, so just get one which costs less, for example is on sale.

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Follow up questions?

  @Alexandre. 

Thank you very much for such a quick reply!

I will check the connection in the hallway and office 2, but as you said, without Deco, it's hard to be sure where the signal will be strong enough.

I appreciate your help.

Have a great day!

  @Pimpek 

Each Deco unit behaves as WiFi router, WiFi signal from Deco should be equivalent strength to an average WiFi router. Yes, that test does not provide 100% guaranteed results, but if signal from WiFi router is good - it gives confidence Deco will communicate to each other well, too.

Of course, you can just order 3-unit set of X55 or X50, place Main Deco and Deco 1 where they should be, configure Deco 2 connect to Deco 1 and check signal strength in Deco app. If weak, move Deco 2 closer to Deco 1.

When you get Deco set and install Deco app on smartphone, if you have questions on tuning Deco mesh by properly placing Deco 2, I can assist.

Make sure you get Deco from the place with return policy. 2-4 weeks should be enough to figure if Deco works for you.