airport express added to wifi 6 network??

IoT stuff still occupies bandwidth even if the traffic is tiny. WiFi has certain limitations.. one of those is it cannot Tx or Rx to/from more than one client at a time. So it must time slice.. and it has to allow each client a chance to talk.. so it must stop the stream and listen for any client that wants a slice. And just keeping everything online requires an amazing amount of traffic. Clients don't sit there ready to work without continual traffic updating everything.

Ethernet only goes from office (upstairs) to the basement.

Are you using another router in the upstairs area or just Express with perhaps a switch.

Running music across your Expresses adds a lot of load as the traffic has high priority even if the data speed is fairly small.

I personally don't particularly like most mesh products. I have everything wired. The whole way mesh products require you to sign up to cloud from manufacturer and use phone apps really is unacceptable. More expensive models like Netgear Orbi can be configured directly.

What I would recommend in your setup is another router in the office upstairs working as AP. You may already do that using the Express but it is not adequate in this role.

So if you replace the Extreme in the basement with say AX3000 then pick up a cheaper AC model.. The Archer C2300 if you can get one would be excellent. Even the Extreme if it is still working ok could be reused in AP mode although they are fairly weak wifi even from new cf latest AC or AX models on the market.

Or you can buy an actual AP.. TP-Link make EAP225 and EAP245 (2x2 and 3x3 AC) .. these are semi-pro level but cheap as they have no router function. It is designed for wall or ceiling mount but you can tuck it away in a suitable corner.. as long as you point it at the majority of your coverage area.

Big advantage of genuine AP is they have the mesh ability to support handoff / roaming so you can walk around the house with your phone etc and the phone will connect to the best AP.

If you wish you could even keep the Extreme in the downstairs location.. assuming again it is ok.. just turn off the wireless.. and buy a pair of EAP245.. Plug one in downstairs to the Extreme and one in the office upstairs.. it gives you all the advantages of mesh with superior ethernet backhaul. Real control to visualise the network. (you can even buy a controller for it. ) This is about equivalent to the Ubiquiti Unifi system but with much easier controls.

just add another AP to the mix and hardwire it??

Yes.. but to gain the advantage of roaming / handoff.. use two EAP245 ..

Would the EAP245/Airport Extreme handle roaming ok?

No.. Roaming uses a whole new set of wireless standards.. 802.11k, v, r as well as a heck of a lot of tweaking. Mixing brands is definitely NOT a good idea. Extreme is 8-9 year old design assuming we are talking the AC model.. and Apple have not updated it since release other than security and bug fixes. Even though the phones etc moved to roaming standards. Hence writing was on the wall for the Apple Airports long before they chopped off the production line.

What I suggest is keeping the Extreme as your router.. that is fine. Turn off the wireless.

Plug one EAP245 into the Extreme upstairs and plug another one into ethernet downstairs.. You can add a small gigabit switch there if you don't have another outlet. They are POE so you could get a POE switch or just use the supplied POE adapter.

TP-Link line of semi-pro products is not well known.. they are obviously trying to compete with Ubiquiti which is also a good solution.. albeit a more complicated product without a built in GUI. In a business situation I would go ubiquiti. They are better at updating products.

The TP-Link are just as good wifi and design wise.. but without the level of support. Hugely easier to configure though.

Here is a good review.. one of the few decent tests I found.

https://dongknows.com/tp-link-eap245-v3-omada-poe-access-point-review/

I do have a set of EAP225 which are slightly cheaper 2x2 version but also very good.

The controller was so cheap I also bought one.. IMHO it works really well.

I have also used Ubiquiti.. but for home use I don't think it is as easy.. though you have more IT skills than most so be worth a look.

Would the EAP245/Airport Extreme handle roaming ok?

Roaming always works best when all of the access points are the same products from the same manufacturer. Even then, some devices like iPhones and iPads will often stubbornly try to hold on the original access point with which they connected, even when they are moved close to another wireless access point.

As far as mixing brands,

Do you foresee any issues with the AP Expresses "joining" the EAP245's for streaming music?

Same answer......this is another one of those times when you won't know how well things will work in your home until you try them out in your home. An outright guess would be that they will work, but it may take some tinkering.

Do you foresee any issues with the AP Expresses "joining" the EAP245's for streaming music?

No.. same as Bob here... Express work OK.. considering their age..

I think the overall improvement in wifi in the house will greatly improve things..

It is getting late on my side of the world.. but as a big believer in experimental science and engineering.. I will do an Express connected to a EAP225 and see how it goes..