If it exists for more than 4 hours, 0 minutes are consumed - the 240 minutes are "refunded" to the pot.
Ephemeral keys can still be used to manage long-lived devices when you want them automatically cleaned up if they disappear - they'll just count as tagged nodes when they hit that 241 minute age.
Note that if an "Ephemeral Device" exists for more than 4 hours continuously, it consumes 0 minutes, and is billed as a tagged device. The docs are being upgraded to change the language around this.
50 tagged resources is the new limit, but there is no longer any limit on user-owned devices, previously, they were bundled together in that 100 limit.
Tailscale services will do that. You can do the proxying with tailscale serve, services gives you the MagicDNS name and virtual IP address bound to it.
Ephemeral keys can still be used to manage long-lived devices when you want them automatically cleaned up if they disappear - they'll just count as tagged nodes when they hit that 241 minute age.