2019 Macbook Pro Wifi dropping to very slow PHY data-rate

VF corporate and two different VF Guest networks are competing directly on channel 36. Your 80 MHz channel there is vulnerable to all that interference up to channel 64.

There are several others including Aco...something there as well (in the range between 36 and 64) that will cause data errors if their packets collide with your packets, so your speeds will slowly decline over time because of competing Networks.

I do not understand what is showing at channel 135, perhaps that is a channel for Mesh networking with multiple Routers?

How many other Routers do you see in the Option-Wi-Fi display?

You could have a situation like this one:

(drag and drop on Preview to see larger or scroll)

this graph from Wi-Fi Explorer, a lost cost Utility with a free trial available on the Mac App Store.

you can also use the built-in Scan display. It will show another Router using exactly the same channel, but it does not show signal spread:

This is available from the Option-Wi-Fi menu

The Tx-rate shown in that "snapshot" has always been the ACTUAL Tx-rate you are currently using.

When you have conflicts with other Routers on nearby channels, it induces errors and cause re-transmissions, and the Tx-rate is automatically reduced until it becomes stable again.

Switching your computer Wi-Fi Off only fools you -- your Mac will start at a higher rate, but quickly revert to the fastest reliable rate, which will be much slower in many-Router areas.

Switching the Router off and on (if and only if channel selection is set to "automatic") will select the least-busy channel. You appear to have good signal (RSSI = 48) so using higher-numbered channel should not be a problem unless you move farther away from your Router. [Distance drops off with higher channel numbers, so everyone wants the lowest numbers.]

If you detect a channel conflict, the usual solution is to change your Router channel to "Automatic". Then when your Router powers up, it listens to the traffic on every channel and picks one that is least busy.

It is not an accident that the first step of every troubleshooting procedure is "cycle the Power on your Router".

In a crowded Wi-Fi environment, you CAN set up a mesh network this way. However, this leaves you open to interference and slowdowns on the linking channel, which looked overwhelmed on your charts:

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The VASTLY preferable way to set this up is to run an Ethernet cable to as many of the Routers as possible, eliminating the crowded mesh Wi-Fi channel as much as possible.

Every well-behaved Router since 802.11g has been able to be connected in this way. Just give them all the same network-name and password, and they automatically merge into one big Roaming Network. This provides higher performance and stability than Mesh Networks.

If you have to install new cables to do this, Category 5e cables up to 100 meters with all eight conductors present and not used for anything else (like Power-over-Ethernet) will allow speeds up to 1 Gigabit. This is adequate for today's standards, but you should consider category 6 or 6a for runs up to about 55 meters. This may allow for up to 2 Gb, 5Gb or 10Gb Ethernet speeds in future.

In the US, cables that run through suspended ceilings that serve as return-air spaces must be flame-resistant "plenum-rated" cables with teflon jacket, or can be any type if run inside a conduit.

I was expecting NSS-index was connected in some way to 10Gb Ethernet-over-copper recently introduced for high-end iMac Pro, Mac-Pro, and an option on Mac mini.

I was unable to find definitive information about NSS-index. The references I could find referred a naming system NSS-MDNS used for broadcast in Unix networks.

NSS may be Number of Spatial Streams, helpful for decoding MCS index for advanced modulation of 802.11ac and above. It is similar to number of antennas used.