This winter, I decided that I needed to do whatever I could to protect my chain, downtube, and bottom bracket from all the salt and road spray.
So, I rigged up a DIY fender extender and have been riding with it for months now.
After every ride, I do tend to wipe my chain and any parts that got sprayed, but I'm blown away by how clean the bike stays simply by having that front fender extender.
The photo shows the fender extender, held in place using duct tape (didn't want to drill holes in the fender, although, some fenders are designed so fender extenders can be put on easily). It's held like this through numerous puddles and all kinds of weather, so "it works". Make it out of a plastic file folder, which I got from Walmart for a few dollars. I can probably make another 3 out of the single file folder.
The only downside I see, which can probably be tweaked by trimming the extender to mitigate it, is that it causes more drag as it acts like a mini sail that's always working against you. LOL I'm not racing with this bike, so it's a small sacrifice for keeping it clean! I'm sure someone more crafty could come up with a design that could be easily removed, although, my setup only requires you to pull the tape off. Easy enough.
That's just awful.
I tend to be the type to say "just ride, or you'll be waiting forever for infrastructure to be perfect", but when it's as hostile and dangerous as you describe, it's super difficult.
Rather than main roads, have you tried looking up alternates to your destination that use side-roads or more residential roads? I've found some very creative routes over the past year that keep me off most of the main roads around me. Doesn't eliminate the use of those roads entirely, but they offer safer options.
There are four viable routes I could take to work. It's 11-14 miles depending on the route. The longest route takes me on some nice country roads with non-existent traffic but every route has some absolutely deadly and busy roads too. There are three large landscaping companies between me and my work and their mulch trucks fly around those blind curves at 50+ mph all day long.