Unfortunately this is not a Traefik issue but a Silverbullet issue. Essentially, the web server doesn't support a subpath and instead assumes that it is at the root - see https://github.com/silverbulletmd/silverbullet/issues/794 for details.
You can try hosting it from a different subdomain or port instead.
I still have a checkbook for the occasional handy man that doesn't want cash or transfer. I'm pretty sure most apps take a cut from business accounts, and others will report to the IRS when you make a certain amount, so for some workers it makes sense to avoid the apps.
Zelle is somehow the one that is usually free and does not report, but my credit union has a daily limit for Zelle transfers, so if the bill is larger, I offer payment in check as an alternative to cash since it's safer.
I think the show does a good job at showing how easy it is for Dale and Bill to swap too. Just recently watched the episode where they basically end up in a ditch because they didn't really know how to use a digger - it's hard to tell who is dragging down who.
You need a domain name that will resolve to the IP address of the machine running the server. It doesn't need to be your own DNS server. If you already have a domain name, you can just add an A record that points to the IP in your local network.