In my split level there is a single interior wall which spans all three levels. Find this wall in your parents house point the installer to it and put the networking cabinet against it.

Plenum rated Ethernet unless that duct is really an electrical conduit and not and airflow duct.

QOS policy being run on Arris G34 router ? Or bad firewall config?

Get a proper router that you configure and then throw the G34 into bridge mode

https://arris.my.salesforce-sites.com/consumers/articles/General_FAQs/G34-G36-Bridge-Mode-Setup/?l=en_US&fs=RelatedArticle

I suspect the problem is that when you put your switch in , it’s going to be faster because of a misconfiguration when you try to use the G34 as a switch

You can configure a router to prioritize small packets (gaming) even if you’re in a high upload or download scenario. I have 120/10 and my son has single digits pings in Fortnite even if I’m doing a video call and screen sharing and my other kid is watching YouTube . This is one way to QoS.