Currently have an Arris  SURFBoard SB6190 and an ASUS RT-N66U with about 36 or so clients on it. I have 3 wired into the LAN ports and then the rest mixed over the 2.4ghz and 5ghz  radios.

House was built in 1984 and is about 1800 sqf but mostly vertical as it has a drive in basement, 1st and 2nd floor and then an attic. Have a mix of cellphones, chromebook, windows and mac laptops, Roku devices, Nest hubs, WiZ lights and Wyze / Eufy wifi cameras.

The modem is doing fine as I bought it July 2020 but the router is getting up there in years. I can't remember the last time I saw an update for it.

Currently the modem & router live on the 1st floor on the east wall and range seems to be ok except the 2nd floor gets crummy reception (except for directly above the router on the 2nd floor).

I was thinking about the ASUS ZenWiFi AC3000 (CT8) but I don't really have a spot to put the 2nd device that would work well on the 2nd floor other than in a bathroom or child's bedroom.

I was starting to look at the UniFi Dream Machine but I don't know what 2nd access point I would need for the 2nd floor.

Anyone have suggestions for this problem?
I don't mind changing brands/vendors but the router must have at least 3 LAN ports by the modem/router as I have a couple of NAS boxes there for the network.

Thanks!

Orbi mesh

MESH

If you don't feel comfortable messing with any configuration this is the answer...If you want to save some $ and have better performance with more configuration and time to set up then go with Ubiquiti.

I have an Asus RT-AX58U.  I just added an Asus AC1200.  

You plug in the AC-1200, wait 1 minute, go into the routers main page, click "Mesh", and 2 minutes later, your range is improved, using the same SSID.

UDM.  You can use any of Ubiquiti's regular access points in wireless uplink mode, though they only support a single hop.  Their mesh branded APs can do multiple hops.

Mesh is okay but the throughput could suffer.

You seem tech savvy enough for Ubiquiti products. Use them.