Blinking WiFi icon in menu bar

It is varying between single and multiple bars, like searching for a WiFi signal.

... and that is exactly what it is doing.

Typically, once a SSID is chosen, the WiFi icon on the Menu bar is supposed to be solid...

In theory, yes, but it is not always the case, especially if there is any form of Wi-Fi interference present in the nearby area. Also, if you happen to have multiple wireless access points in your current network configuration, your MBP may be attempting to roam to a base station with a stronger signal. iOS clients have a similar experience with roaming.

...but the adapter continues searching for better APs or a better signal as shown with the Alt+Click where it varies between 'WiFi:Looking for Networks' and 'WiFi: On' . This is what I have normally seen for years.

Yep, and we have seen this to be more of the "standard" with the later versions of OS X or macOS.

You can do a bit more "investigating" to see just how stable your Wi-Fi network is from the perspective of your MBP by using its built-in Wireless Diagnostics utility. I will get to that in a moment, but I want to take a look at your Wi-Fi values that you posted first to get some clues.

• Channel 112 (DFS, 5 GHz, 40 MHz)

  DFS: This is telling us that your connecting to a Wi-Fi network that is using Dynamic Frequency Selection so that it can co-operate with Wx or military radar. That means it may change frequencies when it "senses" a conflict.

• RSSI & Noise: Signal quality (SNR) can be calculated using these two values. SNR = RSSI - Noise. At this instant SNR = -49 dBm - (-93 dBm) = 44 dB. This would be considered an excellent signal.
  • Ref: AirPort - Optimal Base Station Placement
  • FYI, the SNR Guidelines for Wi-Fi are:
    • 40 dB+ = Excellent signal
    • 25 dB to 40 dB = Good to very good signal
    • 15 dB to 24 dB = Low signal
    • 10 dB to 14 dB = Very low signal
    • < 10 dB = Poor or no signal
• MCS Index:
  • The MCS Index = # of spatial streams + modulation type + coding rate
  • A value of 15 would indicate that your Mac can expect a maximum data rate of 270 Mbps, which matches the value you posted ... but the connection is only using 2 out of the available 3 spatial streams for both the MBP and the AirPort base station.
  • MCS Index for 802.11n and 802.11ac Chart

Ok, back to using Wireless Diagnostics. What we would be interested in is the SNR value over time. Specifically we would want it to not vary much. As you know, you can access this utility via Option-clicking on the Wi-Fi icon on the OS X menu bar. We would want to use the Performance tab. If there is significant amount of variation, the culprit can either be Wi-Fi interference or a faulty base station.

It sounds simple and stupid but have you tried resetting your Mac and your router at the same time?

Just so I understand, is the whole Wi-Fi icon in the menu bar blinking or is it varying between a single to multiple bars?

Signal quality (SNR) looks pretty consistent on both sets of charts holding between 33-35 dB. Also, no large fluctuations in the Noise levels either for both. So far, so good.