@Zac67, you were spot on. Thanks so much! Adding the default route made a difference.
The only additional thing would have been that route entered in the ISP router in order for VLAN 55 to be reached, but the router I have just doesn't have that capability (Google Fiber). I think all the "No" instances in the diagram have to do with this fact, unless I am mistaken.
As an alternative, statically addressing an end host (see PC3) and using the VLAN 1 SVI IP address (192.168.1.102) as its default gateway allows for connectivity.
I have made a new diagram, with more of a complete picture, as there were actually additional goals past the initial inter-VLAN routing, like reaching the Internet from all the networks and so on.
NOTE: In the process of updating the documentation, some end host IP addresses changed but that should be negligible as the original diagram is now obsolete and the networks remain the same.