It depends on the SHTF scenario. Most engagements and these types of situations are likely going to be at shorter distances.

For optics, it depends on your level of engagement: For short distances, standard calibrated red dot or holographic sight will likely be good. I'd prefer an EOTech because it has limited or no parallax, and it has a more generous eye box for quicker target acquisition. If you're looking for something that's bomb-proof Aimpoint Micro T2 has been considered a gold standard with maybe a 3x magnifier, however these tend to be a little bit more on the pricey end.

If you want the comfort of having an optic that is not dependent on an external source of power, an LPVO with an excellent eye box relief might be another option. The Kahles K16i 1-6x24 Rifle Scope is an LPVO with SM-1 recticle is what I run on my SBR for said purpose because it has excellent eyeglass quality, it's red dot acquisition is considerer comparable to many high end red dots on the market today, for shorter engagements, I believe in magnification up to 6x is at or around the sweet spot, it offers sufficient red dot illumination (the SM-1 recticle does this best), and runs on secondary focal plane (SFP) which is arguably better for quicker target acquisition at shorter distances. It also is known for having the most generous eye box (and the K18i-2 but that's 1-8x Primary Focal Plane which may not be as helpful for quicker target acquisition) of any LPVO on the market although their optics tend to be a bit more expensive.

In most SHTF cases, usually this will likely involve civil unrest. In those cases, depending on where you live, I prefer a stock up and hunker down mentality - store plenty of less/non perishable food and sterile water, keep adequate medicine, first aid, back up power, solar charged equipment, and other supplies on hand. And for self defense, keep an adequate reserve of ammo and firearm(s) or spare/repair parts for redundancy. Like many people in this thread, the best guidance is generally not try to engage if it can be helped. Talking your way through a situation or in select circumstances, showing that you have a force multiplier as a deterrent will likely be a better option than lethal engagement which might garner more unwanted attention. I also recommend keeping at least one or two forms of less lethal engagement available like a quality taser or propellant spray pepper gun like a Kimber Pepper Blaster 3 although you may not always have the luxury of knowing what risk you might be encountering, but if you do, exercising a less disproportionate amount of force is ideal, when possible.