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Well, you're starting in a good position as your RAM is double the prod recommended. So that's really good. My org hasn't upgraded yet to 10.5 so I can't give full evidence that RAM isn't affected, but during beta testing I didn't notice any major effect on RAM. And if you do, you could always shorten the vizql session durations from the detault of 30 minutes to something shorter as the vizql process is one of the major consumers of memory.
A rule-of-thumb is 8GB RAM per core. 8 cores x 8GB RAM = 64GB RAM so you look good.
The Hyper data engine in 10.5 takes advantage of RAM so really the more you can use the larger the extract size can be and, basically, processing time will be faster versus a hard drive. If it doesn't have enough RAM it will then use spooling (the hard drive) and that, of course, is slower than RAM so performance will drop but it shouldn't be anything more dramatic than the same situation with the "old" Tableau Data Engine.
A good read: The New Data Engine in Tableau Server