is the mainline situation any better than with ARM?
Unfortunately, sounds like “no” currently. The ones that let you install Debian usually provide some kind of custom Debian image for that specific SBC. Like you, I’m not really a fan of that. But apparently there are some desktop motherboards with RISC-V CPUs coming out. Hopefully that will increase the chance of things getting supported in mainline distros.
It sounds like the answer to “can I run this application on RISC-V” is very dependent on what the backend for that application is. What’s the backend stack for your websites? Are they static HTML sites, or do they have other components? Someone else mentioned that they built postgres and mariadb Docker images for RISC-V, but I don’t even know which programming languages can be compiled for RISC-V right now.