Thinking of upgrading from RBK50 (first-gen Orbi) to 960 / AXE11000

I've actually installed my old (2x of them) 50 series orbis and a friends and parents place. They're still pretty quick and snappy but they were falling behind my homes capabilities. If you're internet speeds are still in the 500mbps (ish) range, the orbi should still handle it fine. I'd try doing a little fine tuning on the 50 series before going to the 960 series (huge buy in) first. They're still kind of at peak prices and 6e stuff will have to start coming down as more manufacturer's bring them to market. 

A little optimizing/fine tuning can help a lot. And I had around the same amount of devices and had 4k streaming. 

What modem is it connected to? 

Have you factory reset the router and reinstalled it (not from a backup) lately? The Orbi has had quite a few firmware changes and sometimes a reset/reinstall can clear out old settings or things causing slow downs. 

You might also try optimizing placement. With changes to devices using higher 4k speeds, sometimes moving a satellite to hardwire in a tv or even just provide better coverage can help a bit. When I first got into mesh systems, I moved my satellites around once a week and tried different satellited configurations from 1-2-3 satellites and their placement. Really let me know how it performed in those different configurations. I found 2x satellites was ideal for my home and 3x was to many. Plus how to place them. 

Thats where I'd start. A reset/reinstall and optimizing placement. I do really enjoy my 963 orbi but I don't notice a whole lot of speed differences than before using my 50 series. The speed difference from 30mbps to 300mbps is for sure noticed but from 300mbps-1gig, I couldn't tell much speed differences unless I was speedtesting. Most sites don't even saturate a 300mps connection much less 1 gig. Even 4k streaming only takes 25-40mbps so unless you have quite a few going at once, the original orbi can still do quite a bit.