Thanks to @slh and @VA1DER for their help... turned out that the solution was a bit more simple. I haven't made any progress on the IPv6 methods of recovery, but the problem is fixed.
On a hunch, I set my computer's IP manually (knowing that the lan itself should be working properly)... and huzaah! it worked.
So the problem turned out to be that the DHCP server crashed.
Fri Mar 17 23:16:51 2023 daemon.info dnsmasq[1]: exiting on receipt of SIGTERM
Fri Mar 17 23:16:52 2023 daemon.crit dnsmasq[1]: inconsistent DHCP range at line 32 of /var/etc/dnsmasq.conf.cfg01411c
Fri Mar 17 23:16:52 2023 daemon.crit dnsmasq[1]: FAILED to start up
And the offending line is here:
dhcp-range=set:Test,192.168.5.2,192.168.5.0,255.255.255.255,12h
Which was the result of leaving the DHCP server setup with a /32 network (I knew the /32 was wasn't going to work, but I was running a test to see what it would do... I didn't realize I would kill dnsmasq by not disabling the DHCP server).
Offending DHCP server config
config dhcp 'Test'
option interface 'Test'
option leasetime '12h'
option start '2'
option limit '1'
Along with the /32 network config
config interface 'Test'
option proto 'static'
option device 'eth0.3'
option ipaddr '192.168.5.1'
option netmask '255.255.255.255'
Once I set the Test network interface back to a /24, dnsmasq was no longer unhappy.