How to protect my new 2023 iMac during a power outage?

I don't use a UPS in my home so I might be misunderstanding your situation here ... but have you seen these:

https://www.macworld.com/article/828279/how-to-configure-shutdown-settings-with-a-ups-on-a-mac.html

Set when your Mac shuts down while using a UPS - Apple Support

Reading these items makes it sound like you can configure Energy Saver to shut down the Mac very quickly after the UPS starts powering the Mac in the event of a power failure. Does this option not work as advertised?

FWIW, I live in an area where storms can, and do, cause power brownout/blackouts. But there are other times other factors can cause these as well. The term we use here is that we have "dirty" power.

As such, I had figured that I had, at least, two options, and they came down to the total cost.

Option #1: Consists of a number of surge protectors, UPS backup systems, and a portable generator.

Option #1: Same as option #1, but replaced the portable generator with an "automatic" fixed generator.

For either option, the surge protectors were used for any circuits that our electronics were connected to. Similarily for select devices, which we keep running 24x7, were protected with the UPS systems.

The first option relies on someone being home, should the power be out for more than 2 hrs, to switch the circuit breaker panel to a sub-panel that was tied to the portable generator. Of course, they would have to manually start the generator. This option was the lower cost option of the two. That cost would depend on the number of surge protectors/UPS system employed + the portable generator. (We use a Honda model for this.)

The second option does not require anyone to be there as the automatic generator knew to start after a specific time has elapsed from the start of the power outage. This is, by far, the more expensive option. Primarily the cost of the automated generator system you will need to power your equipment. Most folks get ones that will power their entire home, especially their HVAC & electrical appliances. (It was -30ºF here today. Since you live in Vermont, I'm sure you're no stranger to cold weather.)

If I were in your shoes, if installing the NUT client will not work on this iMac, I would connect it to the UPS and make sure everyone knows to manually shut the computer down if the power goes out. If no one will be at home, I would shut down the iMac before leaving. At least with my wife's iMac M1, shutting down and starting up again takes just ~ seconds, it is very fast. Occam's Razor says: the simplest solution is often the best one. You can go with more complex approaches that are costly but as the OS progresses, they might not work in the future.

None of my Macs are configured to reboot on power failure. All are on surge suppressors.

In practice, power restoration is initially sporadic and not necessarily stable voltage, so to avoid any potential equipment damage from under/over voltage fluctuations, I wait until power has stabilized before manually powering the Macs back on. Same story for the Synology NAS here.

Depending on the severity of approaching storms, and/or what I see on my RadarScope Pro application as color radar, I may shut the equipment down before the storm arrives.

Yes, we rely on propane for this, as well as, for cooking. Just replaced our 15+ year old heat pump with a modern version that works down to 0ºF, and is augmented with a wood stove when the temps go below that.

Looks like steve626 has provided you with excellent links that may just be what you are looking for.

Thanks for providing the update, and especially about the correct UPS to use in this situation. I'm sure it will help others who have similar requirements. I know I will be looking into this myself.