There are ducted mini split systems that may be useful to you.

Pushing (or pulling) air from an existing duct system is not a great path forward, as balancing the flow is challenging and critical. For example, if you could plumb the mini split into your circulator, what happens if they’re both on at the same time? The mini split could frost over. And if the circulator isn’t on, you could be back-flowing the air intake of the circulator.

Yep the real challenge is understanding when it’s fucked vs when it’s slightly off kilter and a professional can give it a stern look and fix it, but with most things these days it’s always a “well, we can get you a new module, but it’s on back order; it’ll take 6 weeks to come in, maybe buy a new widget”.

In lieu of nothing, there’s UV lights for drinking water that might fit the need that are reasonably reliable… or maybe just a binge of bigclive opening things up might give the key for cracking that UV light. good luck!

“It’s already fucked, might as well try” is some advice I heard many years ago and stuck with me. An educated guess is better than just giving up!

Given the nature of bathtubs and how they can invisibly leak until it’s moderately catastrophic, I would lean towards outright replacement.

I tried for a while to use two 16:9 vertically. Like you say, vertical makes a lot of sense and it works great. But web devs seem universally to assume that if it’s a tall narrow screen, to show the mobile version.