Since you've posted a lot more info, I'm editing my answer to suit your need and those who might have a similar issue in the future.
TLDR: You have good electrical, you just overloaded it. It happens.
The crux of the problem is you were overloading a single branch circuit. You've got continuous loads of 6-10 amps (computer), three 0.5-1 amp (A) monitors, at least 2A of lighting, so 9.5-15A. So far, within the range of a single 20A branch circuit.
The printer, however, pushed your good wiring past design limits. The printer claims 10A draw while underway, which means drawing up to 25A on a 20A circuit. But fusers often have a fraction-of-a-second inrush current of 20A or more. This guaranteed a brownout on any single 20A circuit, and risked a breaker trip.
When you revised your question, you showed us pictures and diagram of the site. There's an outlet right next to the service panel. Electricians typically install those to power their own tools while wiring the house, and they put them on a separate circuit. Moving the printer there is a good permanent solution.
Now, how do you catch this in the future? Get a $20 Kill-a-Watt power monitor. It lets you measure each load (in turn) so you can definitely know the current each appliance draws, and plan circuit balance. It will also show voltage sags, which combined with the known current draws make it easy to distinguish a true overload from a too-long wire run.
When you simply need to draw more power than a circuit can handle, there is no gadget which will let you do that.
Not Surge suppressors. They limit voltage surges, not voltage dips.
Not automatic voltage regulators or line-interactive UPS's. They make things even worse, by drawing more current still to compensate for lost voltage. They are intended for cases where the utility company cannot provide full voltage to your house, and your branch circuit has extra amp capacity. They should never be used to correct voltage sag in an overloaded branch circuit.
Not UPS's. You might think a UPS could store power to help bridge across short-term overloads, but as a practical thing, no. That's not what they're made for.
The tech that works is plain old copper wire. Installing wire is a very learnable skill for tech types, and it's shockingly cheap. The cost is in the labor, which can be considerable if it's behind finished walls.
If you have no other option, there's a bold trick that lets you double a circuit's capacity: Re-designate the circuit to be 240V. Keep the wire, change all outlets, the breaker, and any fixed appliances like lights. Some of your gadgets are already multi-voltage (120-240V). For the rest, use a transformer. For wired-in lighting, disconnect or make sure the fixture is rated for 240V, and use multi-voltage or 240V bulbs. For fluorescent lights, most new ballasts are already multi-voltage 120-277V, check yours and replace ballast if needed. While complicated, this is a solid and safe solution to a hard problem.