the thing with librem v4 is that they only have one slot of RAM and the memory runs in single channel mode. for some that is enough reason to wait for the v5 that promises to fix this.
RAM speed is ussually determined by a simple formula : divide the mhz speed by the latency (ex 13-15-17-19 )
for example : 2400/13 is aproximately 184,6
2666 (mhz) with latency 15-17-19-21 (randomly chosen) - you take the first 15 then divide the mhz with it
for example 2666/15 is aproximately 177,7
the one with the lowest number value is usually the faster RAM module.
also
then visit each RAM manufacturer site to find exactly what mhz and cas latency each individual ram module or kits allow out of the box (you can overclock in the UEFI BIOSES but it’s not recommended if you don’t know what each value influences). then you have to find out what each motherboard and CPU combination maximum (mhz value for RAM is allowed) and you can make and informed decision.
for m2 nvme you have to also take into consideration how much TB of data you know you want your drive to be guaranteed to last for write operations by the manufacturer.
samsung has the pro version for the 960 and 970 that have exeptional endurance and speed but they are more expensive. the evo is less enduring for write operations but is almost as fast and cheaper. the same goes for all manufacturers. you get what you pay for (or so they say 
)