cat5e can handle 1gbe, 2.5gbe, 5gb and 10gbe over short distances...
I don't currently have any firewall. I have ddwrt as my main router.
DD-WRT has a packet filtering firewall, statefull firewall, NAT and proxy functionality...
the switch I linked to is fanless.
your NAS runs software that is neither hardened for nor designed for direct internet access...
synology has had a plethora of exploits over the years... https://www.synology.com/en-global/security/advisory including but not limited to ransomware taking over the nas and encrypting all of your data... and that's just the exploits THEY KNOW ABOUT. synology often takes MONTHS if not over a year to resolve critical issues that normal customers won't be affected by with best practices...
synology's own guidelines clearly state
Do not expose DSM to the Internet unless necessary.
If you must access file services over the Internet, it is strongly recommended that you use a VPN to connect to your Synology device.
direct internet access to your nas is a timebomb. you will lose your data, others will view your data, and you put your entire network at risk by doing do.
you're clients are 1gbe... you can get a brand new unmanaged 24 port gbe switch for $50/£56 a fraction of one of those machines... https://www.amazon.co.uk/Tenda-Ethernet-Internet-Splitter-TEG1024D/dp/B09DPLVLPY/ -- not to mention you can get a used managed switch for less than half that.
if £56 is going to break your bank then get some double sided velcro and clean up your mess. wish you had mentioned your obscenely limited budget...
you can replace the 17 power supplies with a server PSU... these should all be 12v DC input with a standard 2.1x5.5MM barrel connectors...
flip them up onto their face, with the rear facing up. if you want to get fancy you could even make a base that integrates a little wedge and rod for the power button.
get at least a 24 port switch and micro ethernet cables.
this could be cleaned up to a single row on one shelf with no visible wires other than the ethernet and power lead running up and to the back. 1 switch under the machines... 1 PSU instead of 17... even one power cord...
or a little cable management at the very least...
if you're going to buy a no name switch, at least get a better one... https://www.amazon.com/TEROW-2-5G-POE-Multi-Speed-Compatible/dp/B0CGDF35P1