Reddit mentions of JEGS 11023"Aircraft Style" Covered Toggle Switches

TLDR;

Yes SSDs; no HDDs. Watch for data corruption on the SSDs though.

Consider reaching out to vendors and manufactures for discounts or free goods.

If you want devices to stay on whether the vehicle is on, off, or starting, just hook up to the battery.

I found a rugged NAS. Probably not much better than Synology but take it or leave it.

Unqualified thoughts at length:###

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SSDs are the way to go. HDDs will absolutely fail if operated in rough conditions.

SSDs are expensive so do you need a redundancy? You said the data is already replicated/backed up so consider how bad it is if the NAS goes down and all data is lost.

SSDs don't store data well when stored powered off above room temps. Assume your data will need to be refreshed often using a sync method that compares contents, not just modified time. Like linux rsync with the checksum option.

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You're a non-profit (and emergency services to boot). Vendors or manufactures might have programs that will provide a discount.

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A quick google turned up this nas that is designed for your situation, including powered from a vehicle directly. I wonder if it's worth it--I'd guess as almost every NAS probably operates just as about as well (temperature range and dust-wise as this thing, and I didn't see any actual certs on this thing regarding dust or moisture resistance). But one vendor has it for under $700 shipped so only 2-2.5 x as expensive as synology.

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You may not need a UPS if you power it directly off of the vehicle battery. Just run some wires back and hook up an inverter. Though I don't know what kind of vehicle we are talking about, they have them in 12V or 24V and the battery should be charged by the alternator.

Most NASs (and embedded computers like my mini-ITX mobo) run directly off of 12V so you may be able to run it directly from the battery...no inverter. However, get something like this. Make sure to install an in-line fuse or an extra fuseblock fused for the same or less than your regulator. Your all-in-one router (and even some rackmount switches) also run off of 12V.

So I would run a suitably rated wire from the battery to the network area and install a fuseblock as needed. One fuse per regulator, and regulators to take the load of whatever devices you hook up. Look at your equipment specs for total wattage, add it all up and see where you are. A small NAS with SSDs and a dumb 8-port switch can probably be run with one 10 amp regulator and an inline fuse.

Phantom power. You may want to install a switch to cut power to all the equipment so that you don't slow-drain the battery when you aren't using the equipment.

Specific to your equipment##

The synology DS416slim includes a 12V 5A adapter. Your WRT1900 includes a 12V 4A adapter. The regulator I linked above is rated to 10A. I would hook these to a single inline 10A fuse and a 12V regulator rated for 10A, off of 16AWG wire [calculator] from the battery, splice in two of these, and call it a day. Use 12-10AWG if you think you will add more devices to this set up in the future.

Drawing. https://i.imgur.com/VPpe8YC.png The switch can be placed anywhere is convenient. If you run higher rated wire with future-proofing in mind then you might put the switch before the fuse so you can use the same switch to turn off all devices (amperage permitted on the switch).

The rugged qnap nas includes a 7A adapter, so you would need a larger fuse. It has its own regulator so I would add a second fuse (keep a 10A fuse on the 10A regulator) and make sure the wire is specced to 20A (probably 12awg depending on length). Chances are you could run the qnap and the router on a single 10A if using ssds, but might as well build conservatively.

If using the rugged nas with built in regulator.

The fuses are there to keep more than the rated amperage from going down the wire from the battery as well as from over drawing the regulator. So make sure the regulator has it's own fuse and all fuses on the wire combined don't exceed the wire rating. An easy way to do this is to put one inline fuse right next to the battery matching the wire rating (20A, or 30A, or whatever you install) and another fuse to protect the regulator like so. With that inline fuse on the main wire, you could even have 6x10A fuses in a block to protect your devices and still keep the total draw on the line to rated loads.

EDIT I drew the ground connected to the negative terminal. However, it is easier (and probably better) to ground to an unpainted part of the chassis.