On further reflection, with a 500 Mbps line and a Deco E4, you should be seeing fairly consistent speeds of 100 Mbps. I referred to it as a duffer, that's perhaps an overstatement because within its design parameters it'll work very well, just you won't ever get anything faster than 100 Mbps. So if you're seeing 55-68, then you may have both a rubbish broadband connection, and if/when that gets fixed you'd hit the E4's 100 Mbps limit.
So I still reckon that an M4 or higher is the way to to (I max-out at 220 Mbps over wifi from an M4 on a nominal 200 Mbps connection, tech tests suggest it would go up to about 430), but plenty of other choice if you want it. But you might want to check speeds with the hub in router mode for a day or two. Rename the Hub's wifi to whatever your E4 mesh was called, and then whilst you won't get the performance of a mesh system, you won't need to reconnect all your devices but can do a better check on the broadband.
Assuming there's no ethernet connected device available, then to test speeds over wifi you need to do it on a decent quality device that is using a 5 GHz connection (turn off the devices 2.4 GHz if you can), and conduct the test in the same room as the router, but at least six feet from the hub (if you're too close speeds can sometimes drop because the signal strength is too high and the two devices cough and splutter). Also, within the hub's wifi settings, set the 5 GHz channel width to 20/40/80, otherwise you're constraining 5 GHz speeds.
Now, VM won;t accept any wireless speed tests (stuck in the dark ages as they are), but if you're still getting erratic or lowish speeds with the hub in router mode then we can investigate the hub's configuration a bit more, and from that see what can be done.
