VW didn't really play around much with the Mk7.5 over the Mk7, most of the improvements are tech based and are compatible to a degree between the two cars. However they've just done a big upgrade of the T-Roc interior as lots of people criticised the original cheap feeling interior.
To my mind, it's a no-brainer to redesign the Golf's dash/UI. The 8 obviously drives really well - better than the 7/7.5 by all reports. Yet the 8's UI has deterred a lot of Golf owners from upgrading to the new one (I'm one). There would obviously be cost incurred in redesigning the dash, but what is the ongoing benefit to VW of maintaining existing Golf customers and attracting new ones otherwise put off by the digitalisation? Greater than the cost, I'd bet.
As for facelift timelines, I don't think previous examples are all that relevant in this case. If Golf 8 sales tank enough, VW may well be forced to act.
Having said all that from a logical perspective, I personally don't think they will, for the reasons Fred outlined above. They're more likely to let the Golf wither and put more resources into development of the ID line. Risky strategy though, to my mind.