Ignition Circuit Voltage Drop…where to from here?

GOOD that is "well done" solder splices are not the problem

"Generally" just follow the circuit and check wherever you can "get to it."

Your likely suspects are the bulkhead connector, the ignition switch connector, and the ignition switch contacts themselves

One way to "work around" the problem is to relieve the switch of load and use the "ignition run" contact to activate a Bosch style relay, and use THAT to supply your ignition loads.

So....get to test points "where you can

Get a long wire and hook to battery positive, and one voltmeter terminal, so you can "move" your meter.

With key in "run"

Probe the engine bay side of the bulkhead connector on the "ignition run" line and confirm what that is. That reading should be VERY close to the highest drop, same as you measured. "Wiggle" that and see if it changes

Next move to the ignition switch connector. Check the battery "hot" line coming in against the "ignition run" line "coming out.

That right there should tell you a lot.

If the one coming "out" on ignition run is quite different reading from the engine bay side of the bulkhead connector, then the problem is in the bulkhead connector. All that "is there" is a short length of wire and the connector itself.

And, obviously, if the battery line going "into" the switch connector is quite different reading from the "ignition run" then it's right there in the connector and or switch.

Keep in mind these drops "add up" and they can CHANGE depending on "wiggle" and that includes the switch contacts, IE you might have 1/2 volt in the bulkhead, .3 in the two switch connections, and another "few tenths" in the switch contacts

And I hope you understand this is "load driven." More amperage on the load side means more drop. If you unplug the alternator field, the drop will decrease. This ALSO means that the more accessories that you attempt to add on the "run" line, the more drop you will have