Am I over-cautious about power security?

I’m looking for a more stable power supply for my 3d printer. I’ve recently come across the following kinds of switching power supplies.

Generic Switching Power Supply Unit

They seem to come in all sorts of different sizes and wattage ratings and look perfect for powering my power hungry 3D printer. The printer draws large amounts (20+ Amps) of current at 12V and these supply exactly that - nothing more, nothing less!

I just have a few concerns about the safety of using one of these devices. To be perfectly honest, my knowledge of electronics and electricity is very basic but I know enough to treat mains voltage with respect. In general, I stay away from it and stick to low voltages.

My main concern is that I noticed there is nowhere to simply plug a mains cable as an input to the power supply. I take it that you gotta screw the cable in yourself. However, isn’t there a fair likelihood that someone may just pick it up by putting their fingers and short the live and neutral together? The exposed screw contacts look awfully prone to accidental contact with not just fingers, but nearby metallic objects. A built-in fuse won’t exactly help here would it?

The other concern I have is whether it will be extremely dangerous if I accidentally reverse the live and neutral wires? Given that it’s probably headed into a bridge rectifier at some point anyway, my basic knowledge of AC power tells me it’s not too much of a big deal in this case but then again, I know enough about electricity to know that I know very little.

I’m wondering if I am being a little over-cautious here or whether someone at my level of electrical knowledge should be playing around with something like this at all?

A little background

At the moment, I am using a typical 320W PSU for a desktop computer to handle the power workload. However, they seem to need a ~1.5A dummy load on the 5V line to improve the voltage regulation on the 12V line for some reason I haven’t been able to figure out yet.

Through testing, I have found that it does indeed seem to be true - no dummy load: ~11.8V, with dummy load: ~12.1V (these are measured without a load on the 12V line).

The PSU is rated at ~18A for 12V and it seems the printer draws more than to close to that since the voltage dips down to 11.7 - 11.8V during peak power draw.

Obviously, we can see that a desktop PSU is probably not designed to handle strangely large values on the 12V line so I’m a bit hesitant on just getting a bigger desktop PSU.

At the same time, I want to use mains (240V AC) powered heater elements to heat the build chamber (4 ceramic cartridge elements at 100W each).

The heater elements are switched on by a relay board (something like this: KY66-R10S-E2Z1-J16.0K).

I would also like to heat the build platform the same way- using a mains-powered silicone heater mat at 1000W (I have a larger and heavier build platform than most of the other 3D printers- that’s why I need so much power to heat it up in a reasonable amount of time).

The build chamber heater elements are not moving, so I’m less concerned about those.

But the build platform is moving, and I am worried that a broken live wire can basically put the build platform at 240V potential, which can be very hazardous to user, if mains connection is not earthed and protected by RCD.

I have come up with the following ways, which I think should protect against such failure:

Use an isolation transformer, and power all heaters from that.
Include a RCD in my device. This should limit the possibility of electrocution.
Power heaters from a DC power supply. I don’t like this option because of the price of DC power supply at required power levels.
What is the recommended way of designing such a device?

Just go buy a large APC UPS. Does the same thing with a battery bank, no risk of electricuting yourself or blowing your printer up, probably costs around the same anyways.