how big is your house built up?

1k sq.ft-2k sq.ft should require 2 minimal.

If its a double storey, you will require 2-3 nodes minimum.

For the dead spot, I think is wise to get a lower end Wifi7 mesh single pack, or just get a used wifi6 deco mesh to place at the dead spot if speed is not your concern. Since the BE11000 is triband, I assume is working sufficiently to provide you a good backhaul speed.

Think it's best to list down your model with version as well, example "BE85 v1" as this is part of the con of TP-link where certain firmware update does not apply to v1 or v2 hence will have differ result also.

But how come your upload speed so bad result 

sometimes there is no internet connection via cable - do you try with other cable as well ?

Sorry as my question will be a bit out of topic. Talking about BE85, I read a lot of feedback regarding the thermal issue, where it’s hotter comparing with BE65, and some even claimed the internal fan are running very frequently to cool down the router. Do you observe similar issue?

more likely is the speedtest server and your Time ISP routing issue rather than your cable or deco fault.

if download speed is okay, technically the deco,cable,your network card is all working fine. unless you adjust some of the hardware settings on your network card or pc.
also try setting QoS on/off, set limit to 1000mbps for both upload and download.

then go to device management and add the device you are testing with as "High priority" under the deco app, see if it helps.

Your only choice right now is to narrow it down one by one and isolate the issue to see where is it coming from.

Try removing all the nodes and remain one main node first and test it through Wifi and LAN, to see whether is the ethernet port behind of the deco is malfunctioning.

Yes, is going to be very trouble some and cause some network down issue on your end but that deco is not cheap, and I will definitely send in for a warranty if the thing is a defect.

As I noticed there aren't any firmware for BE65 and your BE85 is at the latest version as of now, Build 20241023.

Try swapping both BE 65 and 85 as the main nodes and make sure no other nodes are connected to it.

From there on, test all of the ethernet ports and do a Wifi speed test again to isolate and narrow down which deco is causing you the problem. I highly don't think 2 of it will be exhibiting the same problem. Is either one of it, or the ethernet ports or is an entire firmware issue which is causing this slow upload bug.

It could also be the Wireless settings that are causing this bug, example

320Mhz
MLO
Smart Connect ; 2.4Ghz+5Ghz all banding up together.
Eco Mode

So my suggestion, is turn off all this Wireless settings or LAN settings, and test it one by one.

Everything must be isolated one by one to see whether is a settings issue or a firmware issue. From there you can show it to the TP-Link engineers and pray that they will be able to solve the problem on the deco itself.

Once you managed to get to the root of the issue, you can send in a support ticket to them, they will arrange for a TeamViewer session and ask for access to your homenetwork through remotely and help you to diagnose through SSH and PuTTy. From then on, they will take your case very seriously and work out for a firmware fix ASAP. I've experienced that process once before for my Deco X50 when the Weekly Reports wasn't working at all.