Unstable/inconsistent speed on Orbi RBK 852 AX6000 after upgrade to Spectrum Gigabit

Hi, All -

New member of the community here.

I set up my new Orbi RBK 852 system (with two satellites) last Friday, replacing a series of Apple Airports I had been using for some time.  I had Spectrum cable internet with speeds up to 350.  

Previously, I would get that near the main router or on my Mac hardwired; throughout the house, though, it varied greatly.

The switch to Orbi brought immediately dramatic improvement; I was pulling down 350 pretty much everywhere in our house.

Yesterday,  figuring I'd take advantage of the much better capability of my new network setup, I had my Spectrum internet upgraded to their gigabit service, which required a new Technicolor modem.

Since then, I can best describe my speeds as unstable and inconsistent.

I'm able to sometimes pull down up to 700 or so on wifi when close to the router.  My Mac, which is hard wired, typically does better and hits in the 900s, but not always.  Other times, on wifi, I'm getting SLOWER speeds than when I was at 350 internet.  

Oddly, the internet speed also seems to accelerate more slowly, if that makes sense, when doing a speed test.  Previously, it would almost instantly peg at 350.  Now, it accelerates up and maybe it would get to a higher speed, but seems like it is still ramping up when the test ends.  Other times, it is just slow--slower than when I was at 350, which, again, was almost universal throughout the house before upgrading.  There's really no rhyme or reason to it.  I'm conducting these tests on an iPhone 11 Pro Max and a brand new iPad Pro, so both should be able to achieve peak speeds.

I've restarted the modem, router and satellites multiple times.  I also replaced the ethernet cable that had previously run from the modem to the prior router with the one that came with the Orbi, thinking maybe the old one was not capable of handling the higher speeds.  None of this has seemed to make much of a difference, if at all.

The higher speed is clearly there, but for some reason the wifi network doesn't seem to be handling as well as when I had a lower speed service, often producing slower wifi speeds now.

Any ideas on what else I might try?

Thanks in advance.