How far should I push ram? Just did an upgrade and WOW what a difference!

Just putting this out here and also asking a question in case anybody has dealt with this issue I have. 

I had 32 gigs of ddr4 running at the default 2133 mhz for sometime and photoshop as far as performance (With one glaring issue, see my other thread i just posted.) was fine on the 4k or so canvas sizes i worked on. recently I started getting print commissions that require as much as a 20k canvas which was utterly unworkable.  I'm working on one now and have it at about 8k in which i'll do 98% of the painting and just touch it up at native resolution which is about 20k but even at 8k the brush lag was pretty bad with some of my custom brushes, REALLY slowing down my workflow. 

I recently finally got another 32 gigs of ram for a total of 64 and this made a HUGE difference but brush lag with my custom mixer brushes i use was still pretty noticable but at least workable and only minorly slowed me down. 

After reading that ram speed makes basically no difference in ps i was curious and after a lot of headaches i finally got my ram running at 3200ghz totally stable and I don't know what people are talking about because it made the brush lag again noticably better, sometimes nonexistent on this 8k canvas i'm working on.  I'm just mentioning this because despite what people have said elsewhere i did clearly see a difference and it's worth looking into if you're having brush lag on big documents. 

So my question is, considering PS BARELY uses gpu/cpu  but is an absolute ram hog (Gpu/cpu get about 5% utilization most of the time whereas ram is getting 70ish percent) would upgrading to 128 gb of ram allow me to work natively on gigantic documents like 20k? Is that even worth doing?

What setups do people have for gigantic print poster size documents to get them to run smoothly and paint on them?