Is there anything that can be done to outdoor solar lanterns that will make them more weatherproof and thus last longer?

I battle that one all the time too. I have about 15 of them on my property.
The decorative ones just don’t seem to be designed for longevity. I get 2 years max out of them. I even clean the solar panel to recharge the batteries, have tried new batteries, etc.
I use them as a walk path lighting for an Airbnb space. I have larger motion detected ones mounted high that have lasted 4 years now, but I paid good money for those and I believe those were designed to have more longevity that the decorative ones.
Following to see what others say.

I came across this article that gives some tips. It seems as if the initial usage of cycling the battery is one key to getting optimum performance. Cleaning the panels regularly is another. But the rechargeable batteries appear to only have a life span of 2 years.

Yeah, generally an underwhelming result for a decent idea. I’ve never seen ones that are particularly satisfying. To keep them on all night, they are invariably dim, to keep them cheap, they are invariably poorly designed and built.

IMO they’re not worth fixing. Some nicer ones you can change the battery but after 6–9 months they’re getting corroded and becoming unreliable. Generally better to just replace them.

My experience is that consumer level solar is crap in these lanterns and appliances. They (manufacturers) use low end solar receptors, crappy circuit boards, and they don’t seal the lanterns well anyway.

I had two sets of them fail after a matter of 5–6 months. Crap devices.