ReadyNas slow to display folder contents.

Static IP addresses are convenient, but won't speed anything up.  Also, your local traffic won't be touching the router, the TP-link switch will forwarding between the NAS and your PCs.  So putting in your own router won't speed anything up either, but it would be something that would improve your security (isolating your equipment from the other folks in your building).

What model TP-link switch are you using?  Hopefully it is a gigabit switch (and you are using cat 5e or better cabling to the PCs).  One way to improve performance is to get a "smart" switch, and use an ethernet feature called "link aggregation"  That would let you use more than one of the ethernet ports on the NAS, and get a bit more network throughput when more than one person is accessing it at the same time.

The disk speed will make some difference in folder browsing, but I don't think it will be dramatic.  On the disk side, you'd get the fastest folder browsing speed by adding SSDs and using an advanced feature called "metadata tiering",  Since you have two empty slots, you could switch to that.  You'd want enterprise-class SSDs (consumer-grade SSDs are limited on how many writes they can take before failing - there's a TBW - Terabytes written- spec for them).  2x~500GB should be enough I think.  You will need to destroy your current data volume and rebuild it, so you'd need an up to date backup before you do this.

There are two other things you can try for free.  There's a control to enhance MacOS performance on system->settings->services->SMB that you should make sure is enabled.  You can also try looking at the advanced SMB settings for each share, and disabling "strict sync".  You can do that on just one share, and see if it makes a difference.