Cool, thanks for the reply :) I've seen it discussed elsewhere that the way to have SSL (at least with the News example in the repository) is to run through Nginx.
I've recently discovered the Caddy server (https://caddyserver.com/), which makes SSL and application proxy deployments super easy. Like, 2 lines of config:
domain.com #uses the domain to automatically set up SSL with Let's Encrypt
proxy / localhost:8080 #redirect everything to Arc on 8080
I will say that Arc runs a bit resource intensive, and the slightly slow boot times mean you don't want it to have to re-launch because of infrequent requests. I don't know how well it would work on Heroku.
Also, some VPS services like vultr.com offer $5/mo nodes that have more resources than what you get from Heroku at $7/mo anyway.
I should mention that if what you want from Heroku is their deployment process, you can actually replicate some of it pretty easily with Caddy (though I have not done so yet myself; I plan to soon...)
Specifically, they have support for automatically fetching code from git and running a command in the repo, either periodically or triggered by a webhook: https://caddyserver.com/docs/http.git
That doesn't get you the app ecosystem that heroku offers, but you can get a lot of that pretty easily via docker and docker-compose now.