Is an upgrade in order

WiFi coverage is more marketing hype than reality.  If the entire house has adequate WiFi signal, then upgrading is not likely to make much difference.

The original Orbi product line, including the RBK53 package ("K" stands for "Kit", consisting of one RBR50 ("R" for router) and two RBS50 ("S" for satellite), was introduced in 2016 and is based on radio chips supporting WiFi5 (the 802.11ac WiFi standard formalized in 2008 and amended several times: https://en.wikipedia.org/wiki/IEEE_802.11ac-2013 )

The Product Data Sheet claims it can provision "up to 6,000 sq. ft.".  In actual practice, WiFi coverage is a function of where it is practical to locate the three units, the shape of the building, and building construction.

The RBK753 (again a "Kit") supports WiFi6 (the 802.11ax standard first approved in 2014 and amended as recently as 2021: https://en.wikipedia.org/wiki/Wi-Fi_6  

Both the RBK53 and RBK753 are equipped with gigabit Ethernet (WAN and LAN) ports, so neither is suitable for the emerging "greater than gigabit" ISP speeds that are now on the market (2.5G mainly).

The key difference is the increased efficiency of ax over ac when communicating with devices that support ax.  As of this summer, my home now has 4 devices (out of 50) that support ax.  All those thermostats, security cameras, smart plugs, doorbells, etc. etc. have the original radio chips they came with.  So, switching from an ac based system to an ax based system might enhance the speed of those four devices.  Would the user notice any difference?  I rather doubt it.  A Facetime or Zoom call requires a certain amount of bandwidth.  Having surplus bandwidth cannot make much difference.

Because the WiFi 'backhaul' connection between router and satellites has been changed from 802.11ac to 802.11ax, then the backhaul link will be faster and more efficient.  Customers who have upgraded to very fast internet may indeed notice things connected to satellites are more 'snappy'.

These AX based products do not support the (even newer) WiFi6E (6GHz frequency) WiFi.  Nor do they support whatever the hell WiFi7 is going to be when it gets delivered this year (next year?)  Once again, I have zero devices that support WiFi6E, so installing a WiFi network that supports it does me no good.

Notice that as of March, 2023 there are zero iPhones that support WiFi6E:

https://www.techietech.tech/wifi-6e-compatible-smartphones/ 

My take?  If you are

• loading up on WiFi6 capable devices
• are not satisfied with the performance of the current network (coverage or speed)
• are eager to "move up" to a gigabit connection from the ISP and want "blazing fast" performance from those WiFi6 devices

Then I might start looking around at "what to buy next".  Everybody makes WiFi6 (and 6E) WiFi systems at all price points and with varying features..