I replace the batteries in my UPS every 18 months, and don't try to outlast power outages.

I have everything configured to shut down if the power goes down and stays down more than 5 minutes, which is ~20% of the maximum rated runtime. (I'm using repurposed desktop hardware that loves it's watts as a home server.)

I picked the low number for the reasons you've outlined: even if the battery is severely degraded, it's probably not THAT severely degraded and it's a safe time span to ride out short hiccups, but still well under the runtime limits so that a safe shutdown can happen.

That and I've noticed that, typically, if the power is down for 5 minutes it's going to be down for way longer than 5 minutes, so it doesn't matter and I'm not going to have enough batteries to outlast the outage.

The batteries should not degrade that fast. Get a name brand like Eaton or APC or something like that similar to a Power supply, cheap devices can do harm to your expensive devices. Have a couple of setups where the batteries (lead acid) are 5+ years old and they work ok.

Replacing the UPS batteries is required maintenance.
Compare the cost of new batteries to a new UPS and realise it's the cheaper option.

A UPS is really just for brief interruptions, and to bridge the gap until the generator comes on for extended interruptions. If power is that bad where you live, and uptime is that important, get a generator that comes on automatically when power goes out. Or solar panels and a deep cycle battery array or something.

make sure it's configured for clean shut downs before your battery runs out, auto power up on restoration, and hope it doesn't happen. you will eventually have an outage that outlasts your batteries.

I have a large string of batteries from an old telco office, that runs my rack for 14hrs (calculated, I shut everything down around this time) and that did not last for the 2-3 day outage we had after a storm. Without a generator, you will inevitably have an outage, but if you are prepared, then you can mitigate any damage. use NUT if you need to shutdown or power multiple devices from one monitored UPS

As others have said, changing UPS batteries is required maintenance, and I agree 18-24 months is the typical service life for even high-end UPSs. However, you may want to look into LiFePO4 based UPSs, which can handle many more charge-discharge cycles and often have 5-year warranties. More expensive and potentially not as recyclable as lead acid batteries, but maybe appropriate for your use case.

I was able to adapt my ups to use lithium ion batteries. I bought 2 giant ones that are used for rv solar systems. I tried using the other batteries and even when sitting by idle they loose their effectiveness.

My old server is a HP Microliant.

What I love about it, is it turns back on automatically after a power out.

I don't know what magic is in there, but the power goes out, as it can at times, and when power is restored, the old girl just starts up again.

I'm travelling at the moment, and have it at my parents house under a desk. It's been there over 1.5 years and haven't had to have anyone physically touch it.

When it eventually dies, the next machine will definitely have to have that feature.

Two UPSs? Or better 3?

Tbh that is the coolest setup I've is the OpenCompute rack with 3 UPSs powering the DC rail for the rack. Otherwise two UPS for servers with two redundant power supplies.

How do you monitor your setup now, have you/can you use NUTS?