pete

You're quite bold - I like it ;-) in all honesty, is your requirement mounting an NFS share? As indicated by @chris it really is designed for the local network.
How about using something more suited like a WebDAV share/mount?

You're right - I missed that detail. From the graphs alone it looks as if a process ate up all still free to claim (cached) memory, then the system stalled possibly thrashing until OOM kill intervened - as indicated by large chunks of RAM being freed. Allocated RAM in red lowering and cached RAM in blue rising again.

I don't see a clear indication that you have too low RAM... RAM should be "used" fully at all times and your "cached" RAM value suggest you still have quite a bunch of RAM that could potentially be consumed by applications when they need it.
I cannot clearly see a swap usage in the graphs - that would be an interesting value to judge the overall stability of the system with regards to fluctuating RAM usage.

However, once you notice the problem again, right after you manage to log in, run a "dmesg -T | grep -i oom" and see if any processes get killed due to temporarily spiking RAM consumption. If you're lucky that command might lend some insight even now still.

Also, what if you run a "top" command for a while, what's the value for "wa" in the second line like? "wa" stands for I/O wait and if that value is anything above 5 it might indicate that your CPU is being bottlenecked by for instance hard disk speed.

Played that one only 2 or so years after my mother's succumbing to cancer.
That game helped me im more ways than one - fantastic experience, still can hear some of the musical themes of it in my head as I type this out.

Of course you are right, and this should be noted.
But if you so happen to have Calibre already running via for instance your desktop installation you, may also "take advantage your pre-existing Calibre database" in Calibre-Web ;-)

Take a look at Calibre-Web (github.com/janeczku/calibre-we…) which I've been using for what you ask for quite a while now. As the name suggests it can also take advantage of a pre-existing Calibre eBook Database.

What model is the router? I suspect it is a router your provider equipped you with? In that case, with a 500Mb download bandwidth contract it would be really crazy of your provider to hand you a router with 100Mb ports ;-)
In either way looking up specifications of the router model will help here.

I would not upgrade the contract, even if you go beyond your 50mbit UPLOAD speed you won't be sure that no buffering and hence drop in streaming will happen. Note you have a "500Mb Broadband" contract but the upload is limited to 50Mb. Asymmetric bandwidth is typical for "consumer" internet you mostly consume/download - contrary to "hosting" internet uplinks which typically are symmetric and very pricey since you are typically hosting/uploading.

You need specialised software to make sure you can transmit big, uncompressed real-time data (which video basically is) over the internet. It's basically what Youtube does for its users.
It hosts arbitrary uncompressed video data you upload to it (this is your NAS - which you have now) and then displays that data to users on the web in a compressed, streaming fashion (this is what streaming software would handle - which you do not have yet).

In your scenario issues will arise, naturally.

M500 broadband package boasts average download speeds of 516Mbps and average upload speeds of 52Mbps

So, while viewing media from outside your local netwwork, i.e. via Synology QuickConnect, you're limited to 52mbit speed.
If you're self-hosting upload speed matters alot unfortunately. You will surely need something that buffers / transcodes your media for viewing from the internet.

There's something to that claim. Sending uncompressed (i.e. not transcoded) video content over the internet can easily saturate your internet link.
Do you have CIFS/Samba, in other words Windows Network Explorer access to the files on the NAS via your local network? If so try directly opening a video and look at the network dashboard of the NAS and/or your computers task manager (performance -> ethernet tab) to see to what mbit bandwidth the not transcoded stream amounts too.
Consider that the exact same mbit bandwidth will be needed using Synology QuickConnect to view media from outside of your local network.

If you want to work around all that you would probably have to look into something that buffers/transcodes your media, something like Jellyfin/Plex or the likes. For that you'd have to look into running Docker on the NAS but that'll plunge you into self-hosting very deep very fast and may be beyond your initial comfort zone.

500 fibre connection means it is a 500mbit internet uplink?
Have you checked whether the ethernet cable you're connecing the DS216j to your router is a "Cat5e" cable. If it is a "Cat5" you would be limiting and thus bottlenecking your bandwidth to 100mbit max.

Deep Rock Galactic - I should have probably picked it up earlier. It's such a wonderful co-op game with so many memorable, epic moments happening.
Very charming atmosphere, too.