I live in Florida, I keep two things on hand for my HVAC drain line. First is condensate cleaner (less harmful to the plastic than bleach) which I use monthly and my backup is a Gallo Drain Gun. Even on my worst blockages, 2 shots of the drain gun cleared it out.

My drain has a float switch. If the line clogs and water backs up, it turns off the unit. I also have a leak sensor in the pan that integrates with my security system.

A permanent fix would be a larger drain or a backup drain.

Not exactly a "fix", but a water alarm can ensure an overflow doesn't go unnoticed... but then you've got a battery to remember.

That said, the ones I have under my sink and near my water heater have been running on the original 9v battery for over 8 years now

Do maintenance on the air filters. The water is being condensed straight from the air so it’s not hard water or anything. The only explanation is that enough dust is settling in the water that it clogs the drain. There must be some kind of air filter upstream of where dust can collect in the water line, and that air filter probably needs replacing.

https://a.co/d/59Ryepx

Get some of those ac pan tablets. Or pour some bleach in the pan a couple times per year.

What's causing the clog?

I'm assuming this is a split unit? I don't have one, but I thought there was a float protection to prevent the AC from running if there was too mich water in it. Of course, this would probably only work if the clog was near the unit itself.

Preventative maintenance.

As you said, takes years to go bad.

So take the 10-15 minutes of time it takes to clean the duct, once a year.

And start making a list of things you can do preventative maintenance on.

If you can't put in a float switch, a drain snake once a month should keep it clean.

I have a similar issue. My line is coming from the 2nd floor and there is a pipe in the wall that drains outside newr ground level. The pipe is probably 12 foot long and takes a few bends.

I used a snake to clear the line a few weeks ago. Not entire sure what exactly is causing the clog but like you said, it's probably just gunk.

Now after I cleared the line I was ready to pull the drain out, but the pipe is like 1/2" so the snake isn't really blocking any condensate from dripping down. So for the last few weeks, I've simply left the snake in there. I'm thinking with the snake in there either maybe the gunk won't grow to clog the line so easily, or since it's already in there, all I have to do is wiggle the snake once a month and dislodge anything that might be starting to grow.

The reason I don't pull it out is because with the angle and bends in the pipe, I had a hell of a time snaking the snake up there. But once it was up there dislodged the gunk took all of 5 seconds.

The snake looks to be regular steel which will rust with water touching it, so I've ordered some stainless steel wire which I will run up the pipe ans just leave it there. It's not the beat solution but I don't have access to the source of the condensate so I have to work backwards from the drain up.