Have you actually tried using your phone to see what speed/signal(bars aren't always too helpful since they arent always standardised so an app which tells you might be better) you're getting in those rooms. If you're only going out to the corridor, or the next room you are definitely going to see a drop in speed just due to the nature of WiFi, but it shouldn't be enough to fully degrade the Playstation Portal. Like the bandwidth requirement for Geforce Now Ultimate is under 100Mbps, so the Portal shouldn't have a requirement higher than that considering the screensize and you should be able to get 100Mbps in a room over or on the corridor. You'll probably see some packet loss due to the distance, but that should only be occasional small degradation or stutter not total drops of quality.
I don't have the Portal myself since I have a Steam Deck and a controller that attaches to my phone to do largely similar things, but I can't say I've had such a bad experience on those devices only that far from the router, and that was back when I was using Sky's really meh old router (only WiFi 5). Which makes me think it's partially a Playstation Portal issue. I have seen other people say that the Playstation Portal stutters/degrades easier than streaming the same content in the same location/same wifi on other devices. Not that it helps you that its a Playstation Portal issue, if it is you're either waiting for an update if it can be fixed via software, or as everyone else has said, wire up some access points elsewhere.
Powerline might be a good solution but it's truly not guaranteed. You've said your house is newly built, which is a good start as it means at least the wiring is new, but the other thing that can affect speeds is the powerline adapters being on the same circuit. As soon as they have to cross circuits then you start to lose performance. I think either way you'll probably not get the full amount of your connection to them but it should still be enough for a Playstation Portal. To use them you connect one powerline adapter via ethernet to your router (ie your BT hub) and any other adapters into the rooms where you want the wifi extending (or next to a device like a desktop which you want hardlined into the ethernet port in it).
If powerline doesn't work, then you'd have to either look into MoCA if you have coax wired into rooms (I'd say if you have a new build this is probably unlikely since that kind of cabling is going out of fashion but I figure I'd mention it anyway) or getting ethernet cables wired. As others have mentioned you can use wireless backhauls if you buy a mesh set up, but you're looking at quite a big investment, and realistically you're still going to see a bit of a drop in speed just because obastacles are going to degrade a wifi signal eitherway. The only upside might be that the newer expensive ones can use 6Ghz for the backhaul and that might be a bit less congested than 2.4ghz/5ghz(ie less prone to interference so might keep bandwidth a bit better).
