I know the only manufacturer of such 64GB modules: Crucial (Micron), but Micron doesn't sell new chips to Crucial, thus almost no chance to get them anymore (though few months ago there were some). Kingston (using Micron, Hynix and Samsung) still does not have 64GB.
Soon Zen 6 should become available (well, or not so soon). With 7200 MTs memory controller? I haven't seen such JEDEC modules at all. Are they possible without the additional clock driver (like on 6400 CUDIMMs)?
Interesting, that explains why getting 4x48 = 192gb is around 2400 euros, so 1.2x the price for 1.5x the capacity.
Speed has indeed decreased for the modules you can actually buy, while historically you would get faster and faster modules at lower prices in the years after the introduction is a new DDR standard.