> Can the PHY later handle packets that take forever to arrive?
The PHY does not care at all. It's the job of the upper layer protocols which provide QoS, like TCP and QUIC, to figure this out.
> How would this compare to, say, using the same stuff that the Ham guys use? They were doing IP over slow links decades ago.
IP works over just about any 2-way pipe including serial ports. HAMs use packet radio which uses X.25 for the data link and modem modulation for the PHY: https://en.wikipedia.org/wiki/Packet_radio
The PHY does not care at all. It's the job of the upper layer protocols which provide QoS, like TCP and QUIC, to figure this out.
> How would this compare to, say, using the same stuff that the Ham guys use? They were doing IP over slow links decades ago.
IP works over just about any 2-way pipe including serial ports. HAMs use packet radio which uses X.25 for the data link and modem modulation for the PHY: https://en.wikipedia.org/wiki/Packet_radio