Electrical – Are the outlets unsafe

The problem isn't within the toaster, and adding a "power conditioner" wouldn't help. It's simply drawing a lot of power from the outlet, and the wiring impedance between the outlet and the distribution panel is causing the voltage at the outlet to drop a bit.

In general, the lighting circuits and the countertop appliance circuits should be on separate breakers to begin with, so that an appliance fault won't leave you in the dark with a potentially dangerous situation (hot and/or spinning objects).

You should try to work out which outlets in your kitchen are on which circuits, and operate the toaster on a separate circuit from the one your lights are plugged into.

I think this depends on the quality of your old outlets. A lot of times builders might use the lowest quality for outlets and I have gone through houses that were 10-15 years old and just spent a day replacing them all.

However with GFCIs it is not very common for these to be cheaply made. They would have to probably be quite a bit older than 10 years old to be junk GFCIs. So I would keep them unless another GFCI in the same room fails. I like my outlets to generally match if they are right next to each other but that is just aesthetic preference.

Really though if you are talking about electricity in general (save for very old knob and tub or aluminum) if you touch it there is much more chance of you causing an issue than you preventing one.